5 circuits in a single zone all using 1/2" pex. What size pex should I use to feed the manifolds? Seems like 1" would be right for 4 circuits. Does the feed water velocity have to match that in the circuits (for moving air bubbles, noise etc)? Thanks

it could be anything up to and including 1", depending on your flow rates, distances, loop frictional losses and pump selection.

velocity does not need to match.