A seven channel amp would give you an easy way to set up a second zone with the 990 (outdoor speakers or speakers in an adjacent room) or let you passively bi-amp a pair of speakers, but that's really the only benefit I can see. I suppose you could argue that since the power supply is sized to handle all seven channels that you'd have a little extra current available for the channels you are using for those brief moments when the amp is asked to push really hard...

Your depth and width situation sounds fine. The height is definitely a bit on the low side (only 1/4" clear for the 990 and the 7500). A 7125 would be fine in that space, but I'd certainly prefer to have the 990 or 7500 on the open top shelf.