Welcome to Librarium Online!
I have a tyranid list with no genestealers, but I want to put 4 rending claw weaponsbeast mutants in each brood of 20 leaping adrenogaunts (I only have 4 species in the list). Do you think I'd get objections if I used genestealer models to represent them? I'd rather not hack my stealers up for the arms...
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You know the people you play with better than us. The shops and people I play with, I could proxy a GI Joe and say its a Fex if I wanted. It wouldn't bother me a bit to see a few genestealers in a brood to represent something. Show me an army list and that you've spent the points and I say lets play. Tournaments aren't usually as flexible, I've played and seen unpainted stuff pass but my previous example wouldn't. It really depends on the guys you play with, how are they about that kinda stuff? Good Luck.
Do not meddle in the affairs of Tyranids, for you are crunchy and taste good with everything!