for stealing, you might want to turn off gambits for that person and attack the target until it dies. Then you can turn gambits back on and repeat. The steal from someone with 100% HP is a good start, but like you said, it can cause alot of problems in bringing in too many monsters. The main problem in it i see is the fact that if you have a group, the stealer will change view after one has already been hit, leaving damaged creatures alive long enough to do some damage. Putting a gambit like Foe: HP< 100% -> Attack is a good idea, or you could use something like HP<50, HP Critical, Attack. I like using the first one so I can steal and kill each enemy at at time.

It's fun to play with jobs, but it's best not to let them tie you down. Like FFIII, the best thing you can do is take a look at the resources you have, and figure out best how you can allocate them among your characters. From there just look at the set of monsters you'll be fighting in the area and tailor your gambits according to them. Depending on the situation you might want to temporarily switch a character to a related job class.