Sorry I haven't been around this week due to some RL stuff.

Baou was a spell unique to Zatch, not really a spell class. I think when it was first revealed it was about as strong as a Gigano spell, which makes me think Gigano would generally be unlocked around the 4th slot. Zaguruzemu-powered Baou could beat a Dioga Class, so probably a minimum of 7th for Dioga. 

I don't think we should be getting Shin level spells at all, although it might be because I found the serial escalation that happened in the Clear Note arc to be one of the weaker parts of the series (although it helped that they had to go through extensive training to unlock them). At least, there ought to be a few tiers between Dioga and continent-destroying Shin. 

Just my two-cents.