The "rule" always was 1" per imperial gallon, a value which I always felt uncomfortable with thinking it tends to over stock. Barbs are clean water fish and a packed tank is not going to do them any favours at all. That said, the quality of filtration gear these days is perhaps better than it was when I learnt the "rule", in the 1960's. Whatever the "rule", a packed tank is never going to look good, especially with active fish like barbs. Remember also that "rules" concern adult size. If you stock at 1" per gallon with juveniles, you have problems on the way.
Tiger Barbs tend to be a semi-aggresive species, but in a group that size, will tend to keep their attention on others of the same type. Cherry Barbs are quite inoffensive fish that spend a lot of their time patrolling the bottom of the tank, in other words, they don't typically get in the way of Tiger Barbs.
Most barbs are peaceful enough to keep together as long as obvious size differences are considered.
