I am making renovations to my bathroom and want to replace the faucets in the shower/tub. Do I have to stick with the same brand? Will a different brand not fit the stem or something? Can I replace the stem without accessing the wall?

You pretty mich have to stick with the same brand if swapping out stems. However, you can change the look by using a new escutcheon plate or knob. We can tell you more if you dd details, like why do you want to change a stem.