Just so as to not disappoint Jesse, I'll chime in with my success story of using Tamiya Thinner with ModelMaster Acryl paint.
I have never had any issues with it, either with thinning paint for immediate airbrushing needs or for long term storage of the thinned paint. Although you didn't ask about the paints themselves, I have never had any issues with mixing Tamiya paint and/or X21 Flat Base and ModelMaster Acryls.
That said, if I were in a hobby shop to to buy acrylic thinner for ModelMaster Acryls, I would first reach for the Model Master brand. On the other hand, if you want to get painting and already have Tamiya thinner on hand, got for it.
Andy