The rules on using monsters as PCs are here. However, I'm going to have to agree with Perram in that this could easily get out of hand. Personally, I'd use a druid's animal companion as a template for about how powerful a pet should be at a given level rather than going by Leadership.
To put it simply, it's not. But it's not awful either.
Scribe Scrolls aside, the Blast Rune ability is, at low levels, one of the better damage-based magical effects a cleric has access to based purely on the fact that it requires no attack roll and offers no save. All you have to do is get an...