While pex has the same external size, it is smaller than equivalent copper, so that will play into some of what you need to know when sizing things and determining what will work. One of the benefits of pex is that you don't need to treat it like copper - just don't exceed the minimum radius specified for the material you use. To a degree, the lack of fittings will mitigate the difference in flow, but not entirely.
Body sprays typically need a loop, not T's, to ensure each gets the same pressure. You'd be better off with 3/4" pex if you want to have multiple spray heads.
Check out www.johnbridge.com for help with tiling and your glass block. I really like Kerdi from www.schluter.com. They have some videos on that website that describe Kerdi...a neat system for waterproofing a shower.