Do all magsafe power ports have the same form factor?
Yes - the computer side of things is the same, but the newer cord layout is needed for Airs or when the space around the port is restricted.

The 'L-shaped' MagSafe adapters were just a newer design which meant the cable went backwards instead of outwards.  They should fit just fine and I've used one or two third-party ones in the past.  Just as long as you pick the right power-output you should be fine.  Here's a TUAW article on the matter