I think like everything it depends how flexible you are and of course, on the fish. I like to experiment (within reason!) and would always encourage others to do so, it make the hobby very rewarding!
I bought two young angels hoping they may get on (but having separate homes in mind if they didn't, which I knew was likely and was the case!). One angel that grew big very quickly, from a 50p size to almost fist-sized in under 6 months. I was very surprised given that I had read about people keeping a pair in tanks as small as 20 gallons!
They were in a 125l which was supposed to do them for a few years but it quickly became too small for the bigger angel who is probably now about 8 inches from top of dorsal fin to bottom of pectoral fin. The other did not grow so fast - if i had kept the 125 i would probably still have him in there. Now the larger one is in a 240l and the smaller in a 200l and they both look great as single fish in a community and are brilliant 'centrepiece' fish.
I guess if you're willing to try things out and change them if they don't work then you can do it, but don't get too attached to a pair of angels in a 25g if you're not willing to upgrade or rehome as necessary. My husband gets upset every time I rehome a fish but if I didn't I'd have a billion tanks and no surfaces.
