I would like to know whether a pet can increase its powers and stats (eg strength, hitpoints etc) once it reaches maturity.

Right now I'm feeding my pet (dire wolf) primarily melee weapons. Should I aim for a more balanced diet?

Pets get stronger as you do, but they won't gain any additional abilities once they mature.

Feeding weapons is fine... you'll end up with a strong fighter. I fed weapons/armor with a good percentage of magical ones. Wolf came out with good attack, great defense and a decent aura.

well the best is the mythrillhorn in act3... damn thats a good pet.. with starting Hp of 900!.. but what is the highest level a pet can get? .. is it 100..? and have anyone been able to get their character to LVL100?...

IIRC the highest level obtainable is 99. Read that somewhere (can't remember where tho').