STUD1Y
+1y
brass is fine as long as it's rust resitant and you've got a good seal on it.
just go to home depot, and get a galvanized T fitting.
as far as size, it matters, 3/8 = kinda slow not too bad, 1/2 faster.
if you run 1/2" valves run 5/8" line (o.d) so I.D. is 1/2" for a true 1/2 system.
It'll be worth it in the end.
as far as mounting your valves by the bags, it speeds up RESPONSE time, not flow speeds.