I realised I did not really answer your question about interference. Since telemetry is on 433mhz, RC control is on 2.4Ghz, and Video TX is on 5.8ghz, interference is not as much of an issue, but its still worth taking a bit of extra care not to mount everything on top of each other. Things like mounting your camera or TX very close to your Pixhawk could cause some problems so I suggest that you mount everything as far apart as possible. I mounted the FPV gear underneath my IRIS just because its easier but there should be enough room to mount the TX inside the shell right on the opposite side to the telemetry module.
Here are a few pics how other IRIS pilots have mounted their FPV gear.
FPV TX mounted on bottom of IRIS
FPV TX mounted next to telemetry module
Dedicated FPV camera/TX mount
If you dont want to use a gorpo for FPV this is a great option, design files are on thingiverse
One thing to bear in mind, is that sometimes if you mount your camera and TX very close together you can run into some interference on your video signal at long ranges.
Adding an extra FPV mounting plate
Here an old motor mounting plate is used to attach to the rear IRIS shell screws