I think everyones advice is correct from what i have researched, however in my old system i had a rabbit, yellow and the blue in one tank, they all seemed fine. the yellow is a bit more aggresive then an achilles. The hippo has the largest "roaming" requirements, so with having just the 2 tangs i think we'll be ok???? But believe me, if i'm wrong you will all hear about it
i promise, i may make up some other story tho lol
As far as tank parameters, i'm a dam hawk and test twice a week, i think the stability of my tank is doing so well because the LS & LR is from my old system that was running for a few years...