RSL makes tilted in ceiling speakers. They are fantastic.
You dont have to have the same brand all around although it makes things easier. Most important is having similar output sensitivity and flattish anechoic measurements with well behaved off axis performance. Next is ensuring all channel levels are calibrated withinn 0.5db of eachother with your receiver’s manual setup routine. Last is EQ, but autoEQ should be avoided.
Larger speakers are generally capable of more low frequency output and more efficient. Big rooms tend to favour big speakers. It is worth doing some research (I’m sure you are) and saving up to do it right, once.
Sticking with Axiom may allow you to move your old fronts to surround duty and going with new towers for main speakers.
I should add that adding more sound energy could backfire if you have an acoustically live space. Hardwood/tile with drywall and leather couches for example do not lend well to clear sound at loud volumes. I just visited a home where the owner wanted to upgrade from a Bose cinemate setup. Sucked to have to tell them they needed to budget for throw rugs and new furniture too.... 2 second decay times in that one!!
