So I am starting to confuse myself with all of the different transmitters and receivers. I was looking at the Yongnuo triggers and flashes but once I saw the Mitros+ and that it has built-in radio, it caught my eye. Also, the fact that you can get the Kelby kit for 500 bucks and all it gives you is just too good to pass up (please let me know if you feel the same--you get a Mitros+, Odin, light stand, bracket, umbrella, sandbag, and carrying case). Now if I go with this and say I have the Mitros+ as a backlight and I wanted on-camera fill flash, I wouldn't use the Odin right? Would I just be able to get any flash that has the ability to be master? If I got a SB-700, would that still trigger the Mitros+ since it has the built-in radio?
The SB-700 has no radio capability of any kind, so it won't trigger the Odin transceiver in the Mitros+. The Mitros+ does have CLS slave/master capability, so an SB-700 could be used as a CLS master for the Mitros+, but then you're not using radio.
Probably your best bet would be to put a slave flash off-camera on the Odin (or use another Mitros+ as your off-camera back light), and use the Mitros+ on-camera for fill and as the Odin master.