Both the Powder Blue and Achilles are highly suceptible to Ich and need an 8 week QT to monitor. I got my Achilles from the Diver's Den three weeks ago and he has been in QT since. He managed to get ich 2 days after his arrival. I dropped salinity to 1.009 and he is now doing great, with no sign of ich. He will remain there for the next 5 weeks and then be added to my 300.
I think that putting either of the fish mentioned, in a tank less than 180 to start, would drastically decrease its chances of surviving. Also, you need highly oxygenated and strong flow to keep them happy (more so the Achille's). For your tank size I would stay away from either. These fish also get very territorial; the less tank room, the meaner they get.