Q's for gpu crunching

Sorry if this has been covered a thousand times before, but if I try to do a search, I'll forget and I have to go to work.

I'm giving up on the R9 280x in the Ubuntu rig, got hours into it and still failing. (major Ubuntu noob here)

What I'd like is to put the R9 into an AMD rig with a 7950. Is this possible?
Do I need a "dummy plug" for the 2nd gpu?

The GPU's are for SETI only. so any tips n tricks...

Been out of the game for quite a while. Used to cpu only, GPU's are a new thing to me.

Over 70 views and nothing?
Anyway, psu in that rig is weaker than I thought. Have a little time before new psu shows up. However, the questions still stand. I want to hit the ground running.

Next question. Got a GTX980 in main machine, have the lunatics optimizations installed, where does the config file go if I want to crunch more than one unit on it? I MAY have it figured out, I'll know when current batch of dl'd work units run out.