Not really sure because tiger barbs are known fin nippers but my 3 silver dollars gang up on my african ciclids at times and may easily take out a tiger barb
I don't see why you couldn't put them together. The SDs will trump them by far size wise however tiger barbs can be aggressive and hold their own in a good sized school. I'd start with 8 barbs. JMO
if there is just a pleco in the tank i dont see why not either. i have my SD in my 55 and they do not bother any of my other fish. they actually school around sometime with the giant danios. a large school of the TB will prob reduse the aggression too.