Plakats will be okay with barbs, however I highly recommend you skip on the barbs. Barbs, are one of the more nippy fish that I have ever seen... I've rescued many bettas who had been in barb tanks!
Many bettas are weak swimmers. those that are introduced to filters (that can be heavier flow) may even bother bettas - or some learn to get quicker, and stronger at swimming. Many fish also, adore chasing those pretty flowy things called fins
I successfully had Spartan with leopard danios. No problems, school of 6 danios... but he also was hand raised since he was little, and taught early about other fish being okay. I always added hidey holes for him, so he could escape.
But, I have yet to come across a betta who was that mellow! Most my males are solitary. permanently. forever. not even shrimp/snails. Some bettas also react to colors... white skirted tetra versus a red guppy, for instance.
It alwyas depends
Just like it depends on the betta type... plakat won't be as targeted by nipping, a wild type imported will be more aggressive than say, a betta you had for all it's life and you raised it to be gentle (takes time for that x.x)
There is also the gender role. Males, tend to stake out territory and stay in it. females, by instinct like to travel. Females (not all mind you) are more willing to accept other fish in their domain, than a male.