tieing different gauge of speaker wire together for one speaker
Using two different gages of wire is no worse than just using the thinner gage of wire; switching won't harm anything (as long as the connection is done properly).

As for the minimum gage to use, when I was at MIT I ran 150' of #24 double twisted pair stranded wire from the stereo in my room to my speakers in the courtyard. It sounded fine to me. Quite possibly I had/have tin ears, but I'm sure the results weren't awful. So, unless you're using extremely thin or extremely long cables, you'll be fine. (The humongous, finely-stranded, oxygen-free speaker cables you'll find sold for an arm and a leg are just silly.)