Yes you can, just flip the bias selector to EL34 and plug in a set of EL34's, either Mesa brand or selected aftermarket rated for use with Mesa Amps. You can get EL34's rated for Mesa amps from most online tube retailers, just let them know they are for a Mesa and they will fix you up.
No, unless you trust Eurotubes enough to sell you an integrated set that uses both at the same time.....which I do not trust them. If that were a safe thing to do then I believe most other tube vendors would be doing it. Eurotubes is the ONLY tube vendor that will sell different tubes "matched" to work together. Being the fact that they only sell JJ's that is enough reason for me alone not to trust Eurotubes.
Finding EL34s and 6L6s that run at the same bias voltage would need one (or both) types to be well outside their correct design specs. While this is fairly typical of a lot of modern production tubes, it doesn't inspire any confidence that they will either be reliable or sound good. It just sounds to me like a way of making money out of otherwise unusably hot-running EL34s or cold-running 6L6s to me... but if it works, who knows.
