It's perfectly alright to buy angels in pairs, often they are bought in trios because it's hard determining the sex until it reaches maturity and with three you have a better shot at obtaining a male and female. But considering how large they get putting them in your size tank isn't the best idea. Don't fret though, there are a good number of fish that could thrive in your tank. If your wanting to keep the frogs, try and go for a peaceful species such as Livebearers, Tetras, etc.
I personally love my Emperor Tetras, they are beautiful and very peaceful.
Shop keepers will tell you allot of stuff just to get you to buy, you could ask each and every one of us here and we all could tell you a LFS