Holding yourself just outside a large, powerful, controversial social alignment is a pretty uncomfortable thing to do; people in the ingroup aren't going to think of you as an ally, they're going to think of you as a heretic. And thanks to outgroup homogenity, you're going to find it very hard to distinguish yourself from X from the perspective of people outside it. (Insert your own examples here; I'd add some, but all the ones I can think of are politically sensitive.) If you're loud enough and can get enough people on board early, you might be able to establish yourself as a sustainable group mutually distinguished from X. But I don't think it's possible to reliably keep the groups straight as far as people outside them are concerned: once you've defined your ideology you can't pick and choose the criteria people will be using to make those judgments, so they'll correctly note the similarities more often than not.

Possibly you can exploit the Central Category Fallacy. You copy the whole memeplex X, change what you want and declare it to be an entirely different thing, with its own name, ideology, etc. If someone challenges you that it is the same thing as X, you just point to the different things and say "See? Xers believe this and that, but I most emphatically DON'T!" If someone is interested/pedantic enough to point out the similarities, you can concede that sometimes Xers think like you do. Granted, screening for pattern matching means also no Halo Effect from X-ers and their friends.