Activating Sprint Evo 3d on Boost

I have a Sprint Evo 3d which Boost says they can activate with the ESN. My question is, can this be done while the phone is rooted with a custom ROM? Does it need the stock ROM and if so, is that good enough or does it have to be unrooted too? Right now it's s-off and still hboot 1.4 so I don't wanna do anything to screw that up or have to redo anything, if possible.
Another question, if the phone can stay rooted with a custom ROM, can different ROMs be flashed while it's on Boost? I would assume the answer is yes but want to be sure.

Thanks in advance