You are right - Noah took only two of each clean animal - see Genesis 6:19-20. This clearly includes cattle and all clean and unclean animals. This means he would make the entire species extinct if he slaughtered one animal before breeding took place.
But Noah also took seven of each clean animal and only two of each unclean animal - see Genesis 7:2. This means he could slaughter one animal without making the entire species extinct.
The reason for this is that there are actually two stories of Noah's Ark in Genesis 6-8, written by two different authors and cleverly interleaved so that the result appears to be just one, rather complicated story. The Redactor who edited the final version of Genesis no doubt felt that he would face opposition if he removed either one of the stories, so skilfully interleaved them so that the proponents of either story would find familiar text, yet the existence of two stories is not apparent.
The Flood verses attributed to the Priestly author are: Genesis 6:9-22, 7:6, 7:8-9, 7:11, 7:13-16a, 7:18-21, 7:24, 8:1-2a, 8:3b-5, 8:7, 8:13a, 8:14-19, 9:1-17.
The verses attributed to the Yahwist are: Genesis 6:5-8, 7:1-5, 7:7, 7:10, 7:12, 7:16b-17, 7:22-23, 8:2b-3a, 8:6, 8:8-12, 8:13b, 8:20-22.
The Yahwist reported that on leaving the Ark Noah built an altar and sacrificed one of each clean animal and each clean fowl. This was possible because the Yahwist was the author who reported that Noah took seven of each clean animal onto the Ark.