welcome to the board Nutman!

Letīs see about those questions.
1) Iīm not sure that a full install will make the game noticeably faster. You get the music and the movies (not that many) copied to your drive. Youīll save a few secs when your CD is searching thatīs all I think.
2) Yes you keep all your skills and magic when changing. And yes you start at level 1 in hte new class. There is one setback with this. You wonīt gain any ability points until the level of your new class succeeds the level of your former. And no you canīt go back to an old class unless you reset the game (I think).
3) A level 10 warrior needs a huge amount of exp. points to get to lvl11. Letīs say you changed that warrior to a paladin. He would only need about a 1000 to level up. Changing classes is an easy way to get better characters fast! But donīt forget the magic. If you use a pries make sure to get all the vine spells you want before changing class or you wonīt get them.
4) Bards are good in one way, moon magic. You need a high level of music skill to get it to work. I only use the to let my wizard get moon magic early in hte game.
5) As many of your characters ass possible should go through the barbarian class. You get huge amounts of hitpoints. And if you want the rogue skills from the beginning (recommended the first times around the game) start with the rogue and make him a barbarian. He/sheīll then be as good as or better than your warriors.
6) Ninjas are speedy fighters! They use two swords and throwing. I use 2 of them usually. Change them from barbarians and youīve got invincible fighters.
7) And lastly it doesnīt really matter. Most monsters have pretty good ranged attacks that will effect all of your characters. On the other hand some monsters only affect the lead character.
Hope this helps a bit!