not at all, players do it all the time. It depends entirely on what you are aiming for. more important to consider is matching the cabs specs like ohms, power handling and such. what mismatched cabs are you thinking of combining ?
I have a Bag End 1x15, looking for an 8ohm 1x10, 1x12, or 2x8 to complement it. Greater range and coverage. Something small that can be used stand alone for quiet occasions and such.
You often get phasing problems. Mismatched cabs don't necessarily sound bad, but matched cabs sound better. Another Bag End 115 would work better than anything else.
The only situation where I wouldn't see phasing to be AS MUCH of an issue would be if each cab is getting a different signal, so one is running clean and the other dirty, or one is a low and another is a high.
Just make sure you don't mess up the technical stuff (ohms, wattage per speaker, etc) and shouldn't be a problem.
Just don't fall into the trap of assuming a 115 will put out more lows than say a 410, or 210 or whatever. This is not necessarily the case. You can mix many cabs and it will work, but it may not be good.
Lately one of my favorite rigs is: 8 ohm Schroeder 1212L, 8 ohm Aguilar GS 112 and a GK 1001RBII. Kinda mixed up but great tone and volume.
It's not the brand of the cab that matters so much, as the actual design and driver size of the particular cabs.
So someone like Don from LDS could design one to compliment the Bag End 15"? I should call him and ask. I'm not opposed to getting another Bag End, I just don't want a second 1x15.
I use an Ampeg 1x15 with an Ashdown 2x10. My head is a Mesa. Using gear from different companies isn't an issue. It's ohms (and watts in a general sense of "don't run a thousand watt amp through a cab only rated for 200") you should pay attention to.
Same goes for me. I have used a GB NeoX 210 stacked on top of an Aguilar GS212 for some time now. It's a great stack with tremendous projection and great bottom.
Also remember that if your running parallel with two cabs of equal ohms, each will receive half of the watts from the amp. For example, if you're running an 8 ohm 2x10 (200 watts rated) and a 4x10 (400 watts rated) from a 600 watt @ 4ohms amp, both cabs are going to be receive 300 watts. You could get into a situation where you are over powering the smaller of the two cabs.