first before adding fish either rehome the barbs or bump their number up to 6+, then add other fish.. the aggression will not or should be directed to the other fish.
depends on the rainbow.. should have around a 4 feet tank.. like lots of current and swimming room, so if its a tall tank then no i wouldn't get them.
what i would do:
rehome barbs, blue gourami and pleco (if rupper lip/ bristlenose or any small breed then keep)
do a full out planted tank, dirt substrate.. allow plants to grow etc etc (unless you want to skip this step lol)
then get around 6 boesemani rainbow fish (center piece school)
then cardinal tetra , harlequin tetra or neon tetra school.
get a small breed of cories, 6 for the bottom and you are done
if you want a larger fish then you can throw in an angel and IMO you'll be maxed out.
remembering to wait around 2 weeks in between adding in fish.
