It's a risk. I purchased 2 Ocellaris, and put them in my 20g. They have been in there for about a month now with no problems, but they are tiny babies so that may be way they don't fight.
Also why do you want to buy them at Liveaquaria? Go to a LFS and pick your own. Try to get a small one and a larger one and they probably will pair up.
I had a pair in my nano 29gal. They were ok for about 3 months and started fighting for about a month before they calmed down. They actually share the anemone now....
Got a batch of black perculas from Liveaquaria. They looked awesome. I would try and get the larger tanked bred ones from australia. I tried looking for them from my LFS and couldn't find them, and Liveaquaria has awesome customer service.