ok so im thinkin bout buyin an Epi Valve Junior (small stack) and if i did i would want to change the tubes. one problem, i know nothing about tubes. it says that it has a 12AX7 and an EL84, so these are my questions:
what brand of tubes is the best?
could you replace the preamp/power amp tubes with different style tubes? (eg. switch an EL84 for an EL34)
what tubes would you recommend for metal/thrash/blues/hard rock/progressive?
half asians rule!
I wouldn't recommend that amp for metal/thrash/hard rock/progressive. Blues you got it covered.

You can change tubes out but it all depends on many factors and will require biasing. I know near nothing on this topic, but the stickies that are supposed to be read before making such vague topics as this probably has all the info you need.

If not, google does.
I think THD makes a product (can't think of what it's called off the top of my head) that allows you to use different power tube types interchangeably. Other than that, do NOT use any other power tube than what the manufacturer says is okay. EL84's are not typically found in high-gain amps (most likely because high-gain amps are typically high-power also...EL84's don't produce as many watts). If you're gonna get the Epiphone amp, you will likely need to use a boost/OD/distortion pedal in front of it to get a solid metal/hard rock sound. The tube power section will give you the tube "feel" that is so desirable.