If your ammonia is spiking to one then you know you don't have enough bacteria yet to deal with that volume of waste. You didn't put in much rock did you? I think you need to wait till all the ammonia and nitrite is gone, then repeat the process and see if it reoccurs. Then you'll know your tank is cycled enough to handle fish. The live rock will be useful as a source of life for the tank, so you don't just have to wait for it to 'turn up'. Personally I'd take the money hit and get at least a lot , if not all of the rock in in one go, rather than mucking around adding a few pounds here, a few pounds there, but I appreciate your concern.
I wouldn't go with a cannister, except they can be a useful place to put phosphate remover if you need it, and if you can get a big one and attach the return to a spray bar it can be a good way of getting some 'random flow' water movement. But for actually filtering, I'd avoid them. For 50 gals I'd say get a skimmer. If you were running a 20 for example you can get away with weekly, or biweekly 10% water changes, but for a 50 that is going to start to hurt, and unlike freshwater, it's risky to start missing one here, one there.
For a 50, give a lionfish a miss - just to big. Too big for a 55 really.
How hot do the T5's get, compared to an NO T8 fluorescent do you think?
