can I mix different poly finishes? I have a gallon gloss leftover and want to mix it with a gallon of satin and one gallon of semi to end up with 3 gallons of semi.

Don't even think about it. There is no way to determine if they are compatible and it will be a false economy if you have to do it over. Just not worth it!

I disagree if they are all the same brand you can. just as long as you have enough to complete the job. Make sure you mix it very well. The only problem is if you ever have to do a repair you will never match the sheen.