Good move switching from the MRF-250 to the MRF-300, you will like working with the 300 much better.

Yes, you can use one 950 to talk to many different bases, including an MSC-400 and MRF-300. You can actually talk to 15 different bases each on a seperate address and still use IR from the remote for the TV.

I am not sure what the max distance would be for the RFX antenna though. Running that kind of distance you may want to run something heavier such as 18/4 shielded.

If you really need to go 500 feet, one idea may be to hide another MRF-300 somewhere in the middle (like in a closet) around 250 feet because I believe the MRF-300 acts as a repeater (i.e. it will receive RF from the RFX on the RF input then regenerate a new signal with a bit of a boost on the RF out which you could run to the RF input on the MRF/MSC you will have sitting by the equipment.)

Maybe if Eric Johnson is reading this he would have some input on the maximum distance you can run a RFX and on what type/size of wire, and maybe have some input on using an MRF-300 as a repeater/booster.