You should be fine. When running multiple monitors, its the resolution that lines up not the actual size of the monitor. When you scroll the mouse from one screen to the next, as long as all the screens are the same resolution then going from one screen to the other the mouse should appear at the proper place on the screen (IE if your mouse is at the top of the screen and you scroll to the next it should be on the top of that screen as well.)
During gaming you dont really look at the side screens directly, it mostly just adds peripheral vision so I dont think the size difference should bother you much there.
As far as power for gaming, you will be fine on a 7950 as long as you dont mind dialing back settings like you stated. Overall the amount you have to dial back isnt that great, I can run most games on High with my 7950, BUT I did just add a 2nd one for crossfire. With the 2nd card in crossfire I can max most games as long as I dont push the AA too high.
Oh okay. I guess mouse alignment and game alignment are different. And I was also thinking that since they are mainly for peripheral vision, they won't bother me too much. Thank you.