The problem with small tanks and territorially aggressive species is that the fish that gets chased will need to run OUT OF the territory to escape the other fish. Since a single fish may well take up most of the bottom space in your 10g tank, there's no space for the other fish to be "out of the territory".
Obviously, it will depend on the specie - not all species have the same size territory...
One of the other aspects is that you can't keep two (or probably even three) of these fish in a tank, simply because the aggression needs to be distributed between multiple fish - it's a much better idea to have say 5-6 of the same or similar specie, so that there isn't just ONE fish that gets all the aggro.
So, as a generic answer, I think your LFS is on the right side of correct answer.
Now, having a "spare" 10g tank may be used for other things:
1. You could find a small specie that doesn't require so much space - an immediate thought would be Rams, Microgeophagus spp. or Butterfly cichlids, Apistogramma spp..
2. It's very useful to have a spare tank for hospital or nursery use - when the fish in the big tank has babies, you need to grow them out somewhere.