(1)The Gr. word κτίσεως ktiseōs is used several times in scripture, also the Gr. word κτίσις ktisis is used several times. While κτίσις ktisis is not the word used in Romans 8:19 the word that is used there is κτίσεως ktiseōs. In Romans 8:20 the word κτίσις ktisis is used.
Romans 1:20 use κτίσεως
For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
Romans 8:19 uses κτίσεως
For the earnest expectation of the creature waiteth for the manifestation of the sons of God.
Romans 8:20 uses κτίσις
For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope,
Roman 8:21 uses κτίσις
Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God.
Romans 8:22 uses κτίσις
For we know that the whole creation groaneth and travaileth in pain together until now.
Here are some additional text using those Greek words
Mark 10:6 uses κτίσεως
But from the beginning of the creation God made them male and female.
Mark 13:19 uses κτίσεως
For in those days shall be affliction, such as was not from the beginning of the creation which God created unto this time, neither shall be.
Colossians 1:15 uses κτίσεως
Who is the image of the invisible God, the firstborn of every creature:
Hebrews 4:13 uses κτίσις
Neither is there any creature that is not manifest in his sight: but all things are naked and opened unto the eyes of him with whom we have to do.
Hebrews 9:11 uses κτίσεως
But Christ being come a high priest of good things to come, by a greater and more perfect tabernacle, not made with hands, that is to say, not of this building;
2 Peter 3:4 uses κτίσεως
And saying, Where is the promise of his coming? for since the fathers fell asleep, all things continue as they were from the beginning of the creation.
Revelation 3:14 uses κτίσεως
And unto the angel of the church of the Laodiceans write; These things saith the Amen, the faithful and true witness, the beginning of the creation of God;
Based on the texts that also make use of the word κτίσεως and κτίσις, I'd say the interpretation is not so much convincing as understandable. Though κτίσις looks to be most commonly translated creature and κτίσεως most commonly translated creation, neither is exclusive. Either way I don't see a definitive answer as far as all of creation or humanity.
(1) Englishman's Greek Concordance by George V. Wigram
Also, from BDAG's Lexicon:
κτίσις, εως, ἡ (s. prec. and two next entries; Pind.+).
① act of creation, creation (Iren. 1, 17, 1 [Harv. I 164, 11]; Hippol., p 573 Ref. 6, 33 κ. τοῦ κόσμου; 6, 55, 1; Did., Gen. 24, 4): ἀπὸ κτίσεως κόσμου since the creation of the world Ro 1:20 (cp. PsSol 8, 7; ApcSed 8:10; Jos., Bell. 4, 533). The Son of God was σύμβουλος τῷ πατρὶ τῆς κτίσεως αὐτοῦ counselor to the Father in his creative work Hs 9, 12, 2.
② the result of a creative act, that which is created (EpArist 136; 139; TestReub 2:9).
ⓐ of individual things or beings created, creature (Tob 8:5, 15) created thing τὶς κ. ἑτέρα any other creature Ro 8:39. οὐκ ἔστιν κ. ἀφανὴς ἐνώπιον αὐτοῦ no creature is hidden from (God’s) sight Hb 4:13. πᾶν γένος τῆς κ. τοῦ κυρίου every kind of creature that the Lord made Hs 9, 1, 8; πᾶσα κ. every created thing (cp. Jdth 9:12) MPol 14:1. Of Christ πρωτότοκος πάσης κ. Col 1:15. Of the name of God ἀρχέγονον πάσης κ. 1 Cl 59:3. τὸ εὐαγγέλιον … τὸ κηρυχθὲν ἐν πάσῃ κτίσει the gospel … which has been preached to every creature (here limited to human beings) Col 1:23.—Pl. (En 18:1) δοξάζειν τὰς κτίσεις τοῦ θεοῦ praise the created works of God Hv 1, 1, 3.—The Christian is described by Paul as καινὴ κ. a new creature 2 Cor 5:17, and the state of being in the new faith by the same words as a new creation Gal 6:15 (cp. Jos., Ant. 18, 373 καιναὶ κτίσεις). S. on ἐκλογή end.
ⓑ the sum total of everything created, creation, world (ApcMos 32; SibOr 5, 152; ὁρωμένη κ. Did., Gen. 1 B, 6; 13 A, 2) ἡ κ. αὐτοῦ Hv 1, 3, 4. ἐν ἀρχῇ τῆς κ. at the beginning of the world B 15:3; ἀπʼ ἀρχῆς κ. from the beginning of the world Mk 13:19; 2 Pt 3:4. Likew. Mk 10:6; πᾶσα ἡ κ. the whole creation (Jdth 16:14; Ps 104:21 v.l.; TestAbr A 13 p. 92, 7 [Stone p. 32], B 12 p. 116, 31 [St. p. 80]; TestLevi 4:1; TestNapht 2:3; ParJer 9:6; PGM 12, 85) Hv 3, 4, 1; m 12, 4, 2; Hs 5, 6, 5; 9, 14, 5; 9, 23, 4; 9, 25, 1. The whole world is full of God’s glory 1 Cl 34:6. ἀόργητος ὑπάρχει πρὸς πᾶσαν τὴν κτίσιν αὐτοῦ 19:3. ὁ υἱὸς τ. θεοῦ πάσης τ. κτίσεως αὐτοῦ προγενέστερός ἐστιν the Son of God is older than all his creation Hs 9, 12, 2. πᾶσα ἡ κ. limited to humankind Mk 16:15; Hm 7:5. Also ἡ κτίσις τῶν ἀνθρώπων D 16:5.—αὕτη ἡ κ. this world (earthly in contrast to heavenly) Hb 9:11.—κ. the creation, what was created in contrast to the Creator (Wsd 16:24) Ro 1:25 (EpArist 139 θεὸν σεβόμενοι παρʼ ὅλην τὴν κτίσιν).—Of Christ ἡ ἀρχὴ τῆς κτίσεως τοῦ θεοῦ Rv 3:14 (s. ἀρχή 3).—The mng. of κτ. is in dispute in Ro 8:19–22, though the pass. is usu. taken to mean the waiting of the whole creation below the human level (animate and inanimate—so, e.g., OCullmann, Christ and Time [tr. FFilson] ’50, 103).—HBiedermann, D. Erlösg. der Schöpfung beim Ap. Pls. ’40.
③ system of established authority that is the result of some founding action, governance system, authority system. Corresponding to 1, κτίσις is also the act by which an authoritative or governmental body is created (ins in CB I/2, 468 no. 305 [I A.D.]: founding of the Gerousia [Senate]. Somewhat comparable, of the founding of a city: Scymnus Chius vs. 89 κτίσεις πόλεων). But then, in accordance with 2, it is prob. also the result of the act, the institution or authority itself 1 Pt 2:13 (Diod S 11, 60, 2 has κτίστης as the title of a high official. Cp. νομοθεσία in both meanings: 1. lawgiving, legislation; 2. the result of an action, i.e. law.) To a Hellene a well-ordered society was primary (s. Aristot., Pol. 1, 1, 1, 1252). It was understood that the function of government was to maintain such a society, and the moral objective described in vs. 14 is in keeping with this goal.—BBrinkman, ‘Creation’ and ‘Creature’ I, Bijdragen (Nijmegen) 18, ’57, 129–39, also 359–74; GLampe, The NT Doctrine of κτίσις, SJT 17, ’64, 449–62.—DELG s.v. κτίζω. M-M. TW. Sv.
κτίσμα, ατος, τό (s. two prec. entries and next entry; Polyb. 4, 70, 3; Dionys. Hal. 1, 59; Strabo 7, 5, 5; Vett. Val. 213, 6; SIG 799, 7 [38 A.D.]; PGM 7, 483; BGU 3, 19; LXX, pseudepigr.; Just.; Iren. 1, 5, 4 [Harv. I 48, 2] in gnostic speculation; loanw. in rabb.) in our lit. always (as Wsd 9:2; 13:5; 14:11; Sir 36:14; 38:34; 3 Macc 5:11; EpArist 17; Iren. 1, 10, 2 [Harv. I 93, 3]; Did., Gen. 220, 28) product of creative action, that which is created (by God), creature (created by God) πᾶν κ. θεοῦ καλόν everything created by God is good 1 Ti 4:4. πᾶν κ. ὅ ἐν τῷ οὐρανῷ every creature in heaven Rv 5:13.—Pl., of the components of creation (TestAbr B 7 p. 12, 8 [Stone p. 72] τὰ κτίσματα ἃ ἐκτίσατο … ὁ θεός; TestJob 47:11; Herm. Wr. 1, 18 πάντα τὰ κ.; Sextus 439; Orig., C. Cels. 7, 46, 39; Did., Gen. 109, 25) Dg 8:3.—τὰ κ. τὰ ἐν τῇ θαλάσσῃ Rv 8:9.—τὰ κ. τοῦ θεοῦ what God has created Hv 3, 9, 2; m 8:1; humankind is lord of it 12, 4, 3. The Christians are ἀπαρχή τις τῶν αὐτοῦ κ. a kind of first-fruits of (God’s) creatures (here κ. is to be thought of as referring chiefly to human beings; for a similar restriction in the use of κτίσις s. that entry 2) Js 1:18.—DELG s.v. κτίζω. M-M. TW. Sv.
Arndt, W., Danker, F. W., Bauer, W., & Gingrich, F. W. (2000). A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed., pp. 572–573). Chicago: University of Chicago Press.