If you go with Colt, just make sure it has standard pin sizes or be willing to spend a little extra to get the special pins and or pivot adapters. Some of the civilian Colts were manufactured with different pin sizes, I don't know how many or if they still are.
I have Bushmasters, LMT & Stag. I'm happiest with the Bushmasters. The LMT lower is tight in the magwell, not all the mags drop free, although everything else is fine. The Stag has a very tight fit to the upper, tighter than I would like.
As far as getting a rifle, carbine, free float barrel... it all depends on what you're planning on doing with it.
Yes, all mags are interchangeable. Just stay with quality mags.