Is the Nill grip for SG55x compatible with Swiss aluminum lowers? I see it listed as "for SG 55x and 751" at JDI, but gunfactory.ch says "passt NICHT auf Alu-Lower!" which sounds like a no.

It wasn't a Nill, but I did run into a 55X grip that required modification, for proper fit on an aluminum lower. If it was designed before aluminum lowers were a thing, it may require modification.