You should probably read up on tubes, just remember that for each and every opinion on the subject, there is an equal and opposite opinion. Some guys will have you spend $500 on a set of NOS tubes (unused tubes from the 60's and 70's). Ohters will tell you that the quality of tubes manufactured today has improved drastically over the last few years, and is now approaching the quality of the NOS. Some will tell you that its not that big of a deal - amp manufacturers have designed their amps to work well with the quality of tubes as they are now.
Here's a few real quick answers:
Yes you can use Mesa tubes, but that won't impart any Mesa Boogie qualities. They don't make tubes, they re-brand Sovteks or whatever - I'm sure they test 'em and make sure they meet some standard.
No special tools just make sure the amp is off, and they are cooled off. The power tubes have spring loaded retainer clips that you will have to spread to pull them out.
Generally, an amp will need to be re-biased when changing power tubes. This is an adjustment of a voltage/current draw to a portion of the tube (not the output per se), it has an effect on the tone, distortion characteristics, and life of the tube. The newer Fenders, I have heard, have color coded lettering on the power tubes - replace with the same color Fender branded tubes and no re-bias is necessary. If you are replacing with the exact same brand/model, usually the bias will be pretty close and not need a major adjustment. I tried some different tubes that I had laying around in my Hot Rod DeVille (with a couple mods), including sets of used RCA's, GE's and a couple others, and I liked the original (well used) Sovtek 5881's better. Changing power tubes without rebiasing won't hurt the amp - at worst you won't have optimum tone and shorter tube life.
Preamp tube changes don't require any re-biasing, change 'em at will (with the same type)
