Help! Same brand for speaker and sub?

Mixing brands are ok, but integration is usually challenging. Well I have always found it challenging. REL subs are very highly revered both for their performance and because of their methods of integration.  

Yes, you can pair mid level subs to various brands of speakers. What is important is placing them in the best location. I would recommend a pair of subwoofers and if you get subs that come with room correction it will be better.

The next thing would be bass traps for the corners of the room and you should be good to go. 

Paradigms ARC room correction is well respected and they offer many subwoofers that have arc built in. You may have to purchase the mic separately.