Don't know much about AR's. Can you put anybodys brand of complete upper on a Colt post ban lower?? Are the pins all starndard sized to do this??

If it were anything but Colt, I could say yes.  With Colts, you have 2 different pivot pin sizes to deal with, so it wholly depends on which post-ban Colt lower you have.  

This is a brief description taken from biggerhammer.net about the differences:

Colt owners often refer to having a small or large hole upper receiver (and corresponding lower receiver). Small hole receivers of .250" are the "milspec", but during early production by Colt for the civilian market, they produced rifles utilizing a .315" spec, so as to preclude use of military uppers on civilian lowers. This production lasted until roughly 1993-1994. As a general rule of thumb all Colt "unblocked" lower receivers, are large hole, while pre-ban "blocked" lower receivers represent transition years where occasional variations popped up.

Use a ruler and put it against the front pivot hole on your lower.  While it won't give you an accurate reading to the exact diameter, it will show you being close to .250" or .315".  That is the pin hole diameter.  If it's .250, you are good to use just about any upper out there.  If it's .315, it becomes difficult, but thankfully, there are adapters that will allow you to add any upper to you Colt lower if indeed you have one with oversized pin hole.

Thanks, I'll get out the micrometer and do some measuring.