Is blending power tube brands recommended?

Hello tube owners who are wiser than me!

I have a vintage Fisher 500c and have been running a matched quad of current production EH 7591s in it with fine results. However, I still have the original quad of Fisher branded 7591s that I believe actually came with the unit circa 1965! They still test strong.
So, on a lark I threw the Fishers back in and was quite surprised at how good they sounded! Warmer, much more "PRaT", and just more "liquid" especially in the mids. Soundstage was a bit shallower, they're a bit less detailed, and not as quiet, but I really enjoyed the change. EH tubes are still better in some respects though.
So could I run a pair of each safely and would I get the best of both sonically?
I'm no expert, but yes, seeing that they are all 7951 it is safe to try. 1 EH & 1 Fisher on each channel set up to mirror the other channel.
Yes you can do this but you could wind up with the weaknesses of each rather than the strengths. Just try it and see how you like it
I would think that power tubes would need to be biased wheb they are put in. If you change them, you would need to set bias voltage for the different tubes. I trust you noted the locations of the oeriginal tubes. If your unit is not biased you will need to do so, as you could burn up your power tubes more quickly.