Sometimes - certainly not all the time - mismatched radial tires will handle very badly. My 3/4 ton truck came with bridgstones on it. When I needed to replace 2 of them I got a different brand of exactly the same size and load rating. The truck handles so evily that I took them back the next day and got a more expensive pair of a different brand and it handles much better but still not as good as it did with all 4 the same. I'll probably get 2 more new ones before winter. If 2 different but equivalent radials can handle that badly, I can't imagine what it would have been like if I had mixed in bias tires.

That said, I think 11.5" wide tires on 7" rims probably contributed alot to your wandering problem.