Less fragile: Increasing their SIZE is the simplest way. You could also alter the materials that they're made out of so that they're tougher, but that's significantly more complex. It's possible that if you gave cats inherent skill at dodging that they'd be able to avoid more attacks and thus have more survivability simply by not getting hit; do this with [NATURAL_SKILL:DODGING:some number]
More powerful: Again, increasing their SIZE is the simplest way. The other option is to toy with the materials their claws and teeth are made from to make them more effective, but, again, more complex. Also, it's possible that if you gave cats inherent skill with biting (with [NATURAL_SKILL:BITE:some number]), wrestling ([NATURAL_SKILL:WRESTLING:some number]), and clawing (...Probably [NATURAL_SKILL:STANCE_STRIKE:some number]), they'd be more skilled attackers.
Equip armor: Not directly possible, though you could make a new caste of dwarves that are cats. That'd have some oddites, however, like dwarves giving birth to cats and cats giving birth to dwarves, would let cats do pretty much everything dwarves can including all jobs, cats and dwarves would occasionally marry, female cats wouldn't give birth unless they were married (to a male cat or a male dwarf)... Oh, and I don't think they'd adopt dwarves, so they wouldn't actually be "bonded" to anyone like they sometimes do now.
Also, you'd probably want to remove the VERMINHUNTER tag so the cats actually stay with their bonded dwarf instead of wandering all over hunting vermin, except that if you do that cats won't bond to dwarves automagically like they do now.