Yes and No. Nihilus "ate" planets and such, but we have no idea how itd react to a hive mind such as the flood. In a way, if we were using Nihilus...it might drive him madder. The eternslly hungry devourer stares into the essence of an eternally hungry devourer...that's assuming the flood even let him get that close though. Flood have no problem being sacrifical pawns, as theyre hive mind, so flood ships would have no problem litterally crashing into Nihilus' ship before he could reach the gravemind. Also there's the terror of imagining force user flood. Since force funs through the presence of midI chlorans or such, I dont put it fast the flood to adapt and integrate them into the flood biology. Can you imagine the Gravemind as a force user? Learning every power from those they turn, and getting more powerful as more midichlorans are spread through the flood? Until the gravemind is able to project the force through even the tiniest infection form?