Yes, you might be able to get away with it.

The main problem is that both PB and Regals are notorious ick-magnets. Having multiple ick magnets in the same tank can mean a constant battle against ick in your display tank.

cleaner wrasse and shrimps keep the powder spotless and the regal is in with one cleaner shrimp never have seen a spot on him, very mild mannered to for being in a little tank

You can decrease stress in the tank by using the same rock and water as the old tank. Otherwise if you are introducing two new fish susceptible to stress related illnesses such as ich, it's not going to fair well for any of the tank mates.

I have a regal and PB. Regal was in the tank first, about 6". The PB was introduced about a year ago (I think) when he was about 3 or 4". The PB was a bit timid at first but is fine. I've never had an aggressive PB though, could be that the regal was always in the tank first (had him 8 years).

regal and powder are in separate tanks currently and they cant even see one anther so different water will be for both. the rock from the 175g will be carried over the to 270g