I'm not sure it would be wise to have any other fish in a 20gal with the Kribs. Something like a Dwarf GouramI would probably be ok. What ever you were to choose it would need to be a fish that mostly stays in the upper levels as the Kribs will defend the bottom area of the tank quite vigorously when they breed.
20gal isn't really deep enough for Angels IMO though some people do have them in that size tank. I wouldn't be too keen on mixing any other cichlids with a breeding pair in there. I think an Angel with breeding Kribs will just get beaten up.
I wouldn't go for Tiger Barbs. When the Kribs arn't breeding they would be too much for the Kribs to deal with. It would be very difficult for the Kribs to defend their eggs against a school of barbs when they do breed. I don't think any schooling fish would be a good choice but if you did want to try it go with something more peacefull like Lemon Tetras or Harlequin Rasboras.