You know this really is not a problem, in theory. I say in theory because if the tubes fall within Mesa's specs for idling current and can take the voltages in the amp, mixing brands, alone, is not a recipe for problems.
They likely won't be well "matched" even if same idle (bias) rating, because different brands will likely reproduce a different waveform and thereby sound different, but that alone won't hurt the amp (or the tube).
I have seen this done by friends (I haven't tried it myself; I always had a full set of one brand) without harm and the sound may end up being something unique you may actually like. In fact I have heard some argue that tubes that are not matched result in a 'grungier' tone that they prefer.
As usual, check, or have checked, the idle current to be sure, but if they are in range (and mesa tubes certainly should be unless defective) you should be fine. Your ears might know the difference, but the amp won't care.
Good luck
