yup.
most probably
A rock band will generally contain the key board, one or two guitars and a drum-kit.
No, the game only supports one of each instruments
No, they are two different games and software.
You can't play 2 instruments at the same time. You can get 3 different people and play the microphone, drums, and guitar.
if both drumkits are for the wii, then they should work for eachother, if its two different systems then its impossible.
I don't think the two are compatable
A "classical band of four" is called a quintet, and traditionally uses two violins, a viola and a cello. For two, you can use any two string instruments.
Rock band 2 for PS3 only support one(1) mic, one(1) Drum Set and two(2) Guitars (one for lead and one for Bass) Yes you can play three instruments, but you can't play more than the numbers above.
trumpet / sax
For a classic rock band usually one or two guitars, one bass guitar, keyboard, and drums. For a classical music band clarinets, flutes, trumpets, trombones, tubas, baritones, percussion, baritone saxophones, tenor saxophones, and alto and sopranos saxophones and piano. For a jazz band, you'll need trumpets, trombones, baritones, all kinds of saxophone (all mentioned above), guitar, piano, drums or drum set and a keyboard. If you don't include all the instruments in the band, it will still sound fine. Hope this helps.
Assuming that your Guitar Hero guitar is a PlayStation 2 version and not Xbox or Wii, then yes it is compatible with Rock Band for PS2.