Hello, Steven, welcome!
That Onkyo is a very nice unit, and will pair beautifully with Axiom speakers.
Should there be any harm in listening to stereo music through (a) surround receiver/speakers?
Music in surround is wonderful!
I highly recommend Axiom speakers if they are in your price range.
I don't think your situation is difficult - I think it is exciting!
That is a decent-sized room. Perhaps positioning the speakers will be the difficult thing, but you can always experiment. Unless you are planning to stay in this attic for a long time, I would consider getting small speakers (like the M3 or M22) and perhaps some stands (if you do not have furniture upon which you will place the speakers) so that you can move them more easily later.
How is the beer in your area?
