I was hoping someone could tell me if there are penalties for casting magics other than your primary, or even if you can cast them at all?
It is entirely posiable that a "average ability" mage with the wrong sunsign would be unable to cast his diametric - ever. Learning even the simplest diametric spell can be quite difficult - even life threateningly danerously difficult. however, learning your diametric is exactly what one has to do if a mage is ever going to learn the whole circle and posiably go "grey" at some point.
HMG has some benchmarks for mages learning spells outside their primary convocation. Mages start with only being able to learn any spell with teaching from another mage and then being able to create new spells in the primary, then secondary, etc.