She really was telling you the exact truth. She could be clairvoyant, or maybe she's kept a few before? Who knows.
Tiger barbs do better in shoals, but if you think you had problems with fins being destroyed by fish rot... Think again. The tiger barb will become very stressed if it's by itself and probably die but if you add more they will gang up on the other fish. I didn't even put those things in tanks with large fish that you would think could fend for themselves, like Oscars.
Welcome to the world of species-compatibility experimentation! Fun for you, maybe.. Not so fun for the fish - except the barb(s). They will be well fed, even though I think they might be a vegetarians.
OK. Maybe not vegetarian:
The tiger barb is an active schooling fish that is usually kept in groups of five or more. They are often aggressive in numbers less than 5 and are known fin nippers. If you only keep two in a tank, one will eventually chase the other fish. Semi-aggressive fish, they form a pecking order in the pack which they may extend to other fish, giving them a reputation for nipping at the fins of other fish, especially if they are wounded or injured. They are thus not recommended for tanks with slower, more peaceful fishes such as bettas, gouramis, angelfish and others with long flowing fins. They do however work well with many fast moving fish such as danios, platys and most catfish. When in large enough groups, however, they tend to spend most of their time chasing each other and leave other species of fish alone. They dwell primarily at the water's mid-level. One of the best tankmates for the tiger barb is a clown loach, which will school with the tiger barbs and act as they do, and the tigers act as the loaches do. Tiger barbs do best in soft, slightly acidic water. The tank should be well-lit with ample vegetation, about two-thirds of the tank space. These barbs are omnivorous and will consume
processed foods such as flakes and crisps as well as live foods.
I underlined the important bits for you. And bolded the tl;dr. Who's betting on the barb winning who's betting on the betta? My moneys on the barb dying before it gets a tank mate and killing it if it does. So what will happen next??? find out next episode of
SBSP Celebrity Aquarium Deathmatch!
my ph dropped to 7, there is a massive growth of algae suddenly. 
Guess i better clean my secondary filter. I think i've been over feeding 