Those tadpoles you find early on give you some pretty strong Illithid abilities like making black holes or flying around without any major story problems, at least through the first couple acts. Once you hit Act 3 though, there's this special Astral-Touched one that's worth thinking about carefully since eating it permanently gives you those dark veiny marks, but also some seriously powerful abilities. If you decide to destroy it instead you'll keep your normal look but miss out on those advanced powers.

Your companions might give you some side-eye about using tadpoles, especially Lae'zel, but it won't completely wreck your relationships with them. Crushing the regular tadpoles just means missing out on some useful combat and dialogue options nothing too dramatic. That whole mind flayer transformation thing only happens if you specifically choose it with the Supreme Tadpole near the end, so it's more of an optional path than something that sneaks up on you.

Something interesting about Patch 7, they added these scaling Wisdom checks if you're using lots of powers, but the big story choices still come down to those final decisions rather than how many tadpoles you've been snacking on. Really it's more about deciding if you want those extra powers and are okay with the roleplay side of things that come with them.