All I want to know is why does a Firestarter Imp use ice spells??

The family<>SP thing does work. A marmot is small and weak but fast, but with enough accuracy you can flatten one in seconds, so it doesn't give much SP. A humanoid enemy has a 'medium' amount of health and hits for a fair amount, it's as hard for you to beat as you are for it to beat, so it gives a fair amount of SP. A crab is a walking fortress, armoured to the core and hits like a truck, they're hard to beat if you're not covered head to foot in titanium, so the SP they reward is pretty good. Roselings are physically weak and quite slow moving, they don't have much in the way of health, but they have abilities that can switch you from ahead to dead in a matter of seconds, so the SP they give is quite good.

That's my take on it anyways.