mixing tube brands

When you say "signal" tubes, I assume you mean those that handle low level signals, like driver tubes. In that case I see no reason you can't use a different brand than for power tubes. The one case where you want to stick to one brand is within a matched set, which is typically the case for power tubes within a channel.

I like to mix them all up. Right now I have Ei KT90 output mixed with Tele-funky-ones for driver/inverter and a Rca input.

Triode Lab (Tube de Force) mixes tubes based solely on how it functions and sounds.

Manufacturers have always mixed brands of tubes.