As a rule of thumb, I don't believe anything most pet stores say. They tell you what their distributor/supplier tells them. This is especially true of chain stores. Longfin balloon rams are often called "angel rams," but they're not hybrids. You cannot cross an angelfish with a ram. I tend to avoid the "balloon" and short-bodied fish because IMO it's a physical deformity that shouldn't be bred. I feel the same way about bulldog discus with the super-high bodies. I'll get off my soapbox now.
If your fish is a festae, it probably won't stay peaceful for long. Once it settles in and gets comfortable, it'll probably start to assert itself. I certainly wouldn't leave it in a tank with a dwarf gourami for very long.