I was wondering if it is possible to use RCBS dies in Lee presses and was wondering which other brands are interchangeable with Lee presses

I have Lee's Cast Iron press, and use RCBS dies, and the Lee factory crimp die. Dies with the same size threads will work just fine.

As far as I know absolutely all dies have 7/8"-14 threads and all presses accept 7/8-14 dies.......except for the .50BMG dies...

Yah, they will fit just fine. I use mostly RCBS dies and often use them with a Lee nutcracker style hand press to seat bullets when I am experimenting with loads at the range. This is particularly true when working up loads to find how far I can push it. I could load a bunch from med to over max and test shoot them, but i don't like pulling bullets on the leftovers. Also, I stop at the FIRST sign of exessive presure. Usually severly flattened primers.
Bullet seating seems to work fine in this press (No--tool). The loads don't seem to suffer at all in accuracy.