1. about lvl 16 IIRC, but you can replay in HoF mode to obtain higher levels.
2. I usually dont dual class fighters or barbarians, sometimes i give another class a lvl fighter, barbarian or ranger. Look at the guide to multiclassing sticky for some good idea's, but be very carefull about exp. penalty's
3. Very, very powerfull
4. Yes, they can. The make excelent leaders for talking, have some nice spells (including healing) and their songs are very good at higher levels! Try giving you bard a bow, and have him cast spells, play songs and shoot arrows from the back. Bards are very powerfull in thirdedition rules (NWN, IWD2)
now, if you want a combo that has \both fightrt and a barbarian, i would suggest a dwarf ftr4/barbXX - one of the best offencive-oriented damage-soaking tank builds, if not THE best!