I am in the process of making my own DIY chapter and they are going to be a predacessor chapter of the Ultramarines. So my question would be can I use Tyranid war veterans in my chapter since they are an Ultramarines Predacessor chapter?
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.
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