The text in 1 Cor 3:18-23 is the conclusion of the passage 1 Cor 1:10 to 1 Cor 3:17 which discusses divisions in the church and the pride that some felt because their faction leader was better than another faction leader. This is easy to see because it is a precis of all that has gone before, viz:
- Paul, Apollos, Cephas, Christ - see 1 Cor 1:10-17, 3:1-9
- Wisdom of world & foolishness of God - see 1 Cor 1:18-25, 27, 30-31, 2:4-15
- The argument that Paul develops is illustrated by building metaphors (1 Cor 3:9b-17), class structures of noble vs lowly (1 Cor 1:26-27), oratory (1 Cor 2:1), daily food (1 Cor 3:1-2), farming (1 Cor 3:6-9), etc. This is quintessential Pauline discussion.
Thus, Paul strongly condemns pride and factions in the church and encourages all people to recognise each others' strengths and work together for the common good. Perhaps 1 Cor 4:1 ought to be regarded as part of Paul's conclusion or at least a segue to the next section; however, 1 Cor 4 does allude to the earlier three chapters (eg, 1 Cor 4:10).
Thus Paul's comments are directed to the whole church to prevent division and factionalism.
Lastly, let me discuss the details of your final question. I do not believe Paul is debating the equality (or otherwise) of different apostles and their teaching. He is definitely not saying that (v22) that Paul, Apollos and Cephas are equivalent because the list is much longer - he is definitely not suggesting that Paul, Apollos, Cephas, the world, life and death are all equivalent. Paul is discussing the Christian "freedom" as a final argument against factionalism.
The sentence "All things are yours" occurs twice (v21 & v22) and so is important. Because it is part of Paul's concluding remarks about factionalism, one assumes that his list of "Paul, Apollos, Cephas, the world. Life, death, the present or the future - all are yours" means that Christians should use whatever is useful and beneficial wherever they find it; that is, we should not be confined by a faction or division (or factional leader) to which we may be foolishly attached. Paul utters similar sentiments later in 1 Cor 6:12, 10:22, 23, "all things are lawful for me but not all things are beneficial".
Basically what I'm asking is what is included in "all things are yours". Does he include more than just the apostles? For example, does he include R. C. Sproul or St. Augustine?