I'm trying to balance it with the magic items rules. We all know Thaumaturgy can be easily imbalanced. Especially in games with large time jumps(like the one I'll be in), so I'm working off another system, the slightly less broken enchanted items. Now, a summoned creature doesn't have a limited number of uses, so I had to balance that. Having control spells does that easily, and makes it so a summoner needs to use all the mage skills(Lore, Discipline, and Conviction.) making them feel more magey.
The conviction score is based around the idea that summoning and controlling somethings difficulty is completely based around its conviction score. If I allow itty bitty conviction scores then the creatures are simple to summon and simple to control. That strikes me as exploitable. I don't want my character summoning trolls as minions. This is supposed to represent somebody who's whole deal is summoning things, while staying balanced. Trolls are unbalanced when they can be acquired easily. This system limits that by placing a cost.
Demense is for flavor. And has some fun tagalongs anyway. There has to be a place to hold the things. Demense represents that.
Long term I can actually see the character becoming less useful, as he can only control so much at a given time and summons can never reasonably top 8 or 9 refresh. I'm working on that... What it comes down to though is that this feels balanced in a way the other rules don't and making a creature is really easy(for anybody that can manage character creation). Anything from the book, down to pixies is nigh impossible, but flavorwise this feels right.
This does cover skill cap. The creature has to follow the pyramid the same way a PC does. Thus if I spend 4 slots to get 16 skill points(with my 4 lore) I can get a 5, a 4, a 3, a 2, and a 1. However, that creature can really only be used for one thing-A huge limitation. I'd have to have 21 skill points to hit +6 and once again, the summon is still vastly limited. I could always say the creatures skill cap is the highest level skill it's controller possesses to, stopping it at +5. That wouldn't be outside the realm of reason.