my girlfriends dad has a 55 gallon tank and all he has in it is 7 danios 5 red eye tetras and a pleco would it be ok to add some tiger barbs in this or something of a similar size he wants add a bit of colour
I would advise against Tigers as they are unpredictably nippy. Something like gold Barbs, Boesemani rainbows, Harlequin rasboras are all brightly coloured and far less aggresive alternatives.
Yes with my experience with Tiger barbs they are best to be kept in a same species tank. They are little terrors and are likely to pick on the other fish.