This is much closer to the method I'm working on (Archetypes being applicable largely across the board), so I'm rather digging it.

A brief overview has me worried the Sentinal's damage reduction is to high though (especially a Sentinal/Brute). With E6 damage values I might make it just match the Gladiator's damage bonus. That would also have the nice effect of making a trained Gladiator able to basically ignore a trained Sentinal's ability to resist damage, and vice versa on the Sentinal's defense bypassing the Gladiator's damage bonus.

Alternatively, make both the Sentinal and the Brute have a 1/2 level damage reduction progression. Seems the cleanest to me.