Since as small Mario you can't destroy blocks without shells and can't fly, certain levels are impossible to beat. The impossible levels are 6-5, 7-F1 and 8-F. A lot of other levels also have different strats, I'd recommend you to watch a run to practice the small strats. So let's go over the Small route.

World 1, 2, 4 and 5 are the same as in normal warpless. In world 3 you play 3-6 and grab the cloud in 3-7. Then you just cross the bridge to 3-8. In world 6 you play 6-6 instead of 6-5, everything else remains the same. Go for 6-10 if you don't want to wall jump in 6-9. World 7 sees the most differences. Play 7-1 as the devs intended or go for a very difficult floor clip. You cloud 7-4, 7-F1 and 7-6. 7-8 is a lot harder as small Mario, even with a star, so you could go for 7-7 instead. You then play both 7-9 and 7-F2 and hop on the airship. In world 8 you cloud 8-F.

Since you can't damage boost for p-speed strats, stars are also a bit more important. Before world 8 you only get 2 stars if you had good RNG. The best levels to use them on are 7-2 and 7-8. If you get a third star or play 7-7 instead of 7-8, the third best level is 6-F2. If you get even more just use them wherever works best for you.