Sorry if this has been asked before, but I couldn't find the answer in a hurried search (and this may not be a simple one question, one answer):
But is the white cube with black caps competition legal? I've read the rules and common sense tells me it is legal, but you never know......
No problem. I suppose you wanted a better explanation so here it goes: The only way the cube could be illegal was if you sneaked in some regular full black or full white pieces (so that you could determine what stickers are in each piece without actually seeing them). Multi-coloured cubes are illegal because you can see all the colours of the pieces when slicing the puzzle.
In this particular puzzle all the pieces look exactly the same from all angles an there is no advantage from slicing the puzzle in terms of lookahead compared to any other legal puzzle. All the pieces are also put together in the same way so all the pieces "feel" the exact same. Therefore there is no advantage e.g. when doing BLD solves.
55mm Zhanchi or 54.6 ShuangRen are probably the best. I would say ShuangRen, and mod the centers slightly, because I feel like it's more forgiving which I find good for OH.