They should accept parts. The only reason they insist on their parts is for warranty purposes (and to make lots of cash...)... They might dick you on the job or repair warranty, though, so ask them first.

I've been in that situation, too... having a big car that I really like, but was really burning a hole in my pocket. You'll feel bad downgrading, yes, but after a while, you get over it...

And the most joy a car can give you is to be economical and reliable, so a newer, smaller car will make you very happy...