Mixing tube brands

If you do try that make sure that you re-bias your tubes so that they at least match in that respect. If you can't do that don't try. Even then I wouldn't do it.

Personally speaking I generally select matched power tubes of my choice based on SQ (not so much power ratings) and make more subtle changes to their tone by selection of small tubes.

Your power output tubes (and small signal pre-amp and power amp input & driver stage tubes) had better match in electrical characteristics (gain, mu, etc.). If they don't, your electronics will create more distortion than if they do. Read up on how a tube circuit operates.

I would call Uncle Kevin at Upscale and see what he thinks.

depends on your amp and how bias is controlled.  My amp has great bias control and I should be able to adjust the amp to use them in symmetrical pairs so long as they aren't far apart, and IIRC, I've used both of those tubes and they were similar.

I am a bit OCD and always try to have the same brand tube in the power bank, then in the pre amp the same brand across the bank/type.