[K]

Slaves have to be just over half a screen(ish) to be safe from being used since they bunch up. tend to keep mine on the top left and mage on the bottom left.

As for slaves catching fire....Slave matrixes is the route unless you are willing to cast fire protection spells(and even then its hard to get 100%). Use them fast or lose them (though casting Regen spells like Mass Regen would also cure your problem since burning slaves is OK as long as they don't die, and considering Fire's synergy with Nature it's a good investment).

Abysia is pre-built to use Soul Contracts since they have astral mages who can cast Returning when horrors show up, so I'd focus on that and use the slaves for times when you need a communion for a big Fire or Earth spell or something.