Re: Will 7.1 sound good?
Without seeing your complete room layout it is hard for me to visualize other possiblities. Might I suggest that you align the rear speakers with the rear of the couch? Is there glass behind you? If not you may want to experiement by angling the rear speakers a little toward the back wall and use reflective sound. But that gets mighty tricky when it comes to EQ and time delay.
Based on the materials that you stated you had in your room and the rough dimensions presented, I calculate that you have a reverberation time of about .8 seconds at 1kHz, which isn't that bad. Higher frequencies will reverberate a little longer but not much. Lower frequencies should have less reverberation time for the most part. So at 1kHz the decay time for sound is about 75db/sec. Do I definitely think that you can work with that in the EQ.
First, get yourself a center channel and a subwoofer. Matching all your speakers with the same brand is not necessary. Unless you are buying super high-end speakers they are not going to matched pairs anyway. Velodyne has some good subs. It all depends on your budget.
After the center and sub are connected, the Onkyo receiver you have does have audyssey EQ. The Auto Calibration measures sound from 3 positions: listening position, at least 3 feet to the left of listening position and at least 3 feet to the right of the listening position. Try this first and see what you get. Some people say that the auto EQ of the receiver is not good at all. Personally, I have the TX-SR674 and the auto calibration had a great impact on my apartment. But it is different for every space. You can also manually set the EQ within the receiver. To do this, you will need a computer with some Frequency analyzing software. Bob Hodas recommended some earlier (ie: Smaart, EFT, etc.) You will also need a calibrated mic. You may be able to get away with the included calubration mic that comes with the Onkyo receivers. Otherwise, find yourself a good omnidirectional mic. Feed the test signals into your system. It should give you an idea of which frequencies are problematic. You can then use the receiver's EQ to make adjustments to those frequencies. In the end, nothing beats professional calibration. However, with experimentation and careful attention to detail you can yourself pretty close. Close enough if you are not picky.