A picture is a thousand words, or at least one monster.

[Malcadon]

I almost forgot about Brocks dead beat parents and Ash's missing father. Either way I'm not planning on using the official Pokemon world. It'll be a homebrew with normal animals along with select monsters from Pokemon, Monster Rancher, Huntik ect. Also the minimum adventuring trainer age will be around 14, with the PCs likely older than that. I explained it like this on another site. It's right here.

So, I take it you are just going for the beastly, non-human types from the other fictions? Some of the more human-looking monsters in Monster Rancher and (especially) Digital Devil Story* would have implications that would go beyond the notion of just using them as attack animals. Although, considering the dark context of Mon settings in general... In for a penny; in for a pound.

*(by the way, I just love the art style of this this KAZ guy, who worked on the monsters in the early DDS games. Much like Yoshitaka Amano with Final Fantasy I & II, his work can easily be placed among D&D art, without people saying they look too "anime.")

[Agrippa]

Then I'll divide these monsters into two categories. Vassals, which willingly serve you out of loyalty and respect for your strength, and contractors, creatures you have to bargin with and provide services for you in exchange for pacts and deals.

The reason why monsters become vassals is simple. You defeated them so now they want on your side. They will gladly fight along side you and perform tasks for you if requested. In exchange they demand fair treatment and will not remain loyal to you if you demand or request them to perform overly difficult or morally repugnant acts. An individual vassal monster counts as a single hireling for the purposes of the master PCs maximum number of hirelings.

Contracted monsters want something more for their services. Souls, blood, land, gold and other wealth or shrines in their honor. Or they may demand that perform a quest of some sort to prove your worth. This is assuming they don't vassal or contracted monsters of their own. If that's the case tough luck, you won't get them. And yes, you can try and buy them off their current master.Your typical Pokemon would be a vassal while Monster Rancher's pixie would be a contractor.

Maybe we should start a new thread for this.