Actually the team said that the reason Getai slingers don't have AP is because historically the Getai were more fond of archers and the team wanted to encourage the player to use getic archers over the slingers.

As for the Gaesatae. There is a very nice picture on the last page of the screenshots for 0.81 thread concerning them. You might want to check it out Little Legioner. It's hilarious.