i would not put a simple T inline with a keg for a counterpressure filler because if the filler ever was opened, all the head pressure in the keg would quickly escape, wasting gas. if you wanted to T that line and just remove the ball or pin lock connector while you are using the bottle filler, that would be fine. just dont use them both at the same time. if you had a check valve between the T and the keg, then it wouldnt be a problem.
if you want to make a manifold to go from one regulator to a few kegs, you can certainly use Tees and crosses instead of buying an expensive manifold. a manifold with ball and check valves on each hose is a nice thing to have though.