Tyranid War Vets Questions?

The Ultramarines do not have predecessor chapters. They're first founding .

That being said, the issue is whether you're using one of the accepted rules sets. Ultramarines have no traits and no deviations. If your successor chapter follows all of the same rules that the Ultramarines do, exactly, I don't see any problem with you playing Tyranid Hunters. It's when you try to mix rules--say, take Tyranid Hunters along with traits or whatnot--that it starts becoming functionally illegal.

Tyrannic War Veterans can only be used in a straight up Ultramarines army. Unless of course, you play a DIY with no variations, and your opponet lets you feild them.

Im not to keen to let people do that, but as long as your oppenet is fine with it, and your army is stricly codex, it would be fine.