taga can be anything up to 32 bit dude. For a texture you will get a good depth at 16 bit but it will only make a difference if you create it at that depth, so if you paint a texture in the PS default settings of 8 bit then it wont make a difference if you up the bit dpeth to 16 before saving.
I allways create alpha channels seperately, but im just used to working like that, you could allways use maya nodes to generate the alpha i guess, e.g. using the reverse node to negative your values and then pipe the result further down the network.
FX supervisor - double negative