Bards. They should all be bards
This is a possibility.
You may be onto something - or possibly, to reflect reflect their Herald Powers not being the same sort of magic as wielded by the Usual Suspects - Warlocks? And reflect their specialisation through stat allocations, possibly multiclasing.
And HUmans - one, because there are no non-Human Heralds as far as I know, and two - that extra Skill to show how well they were taught at the Collegium.