Ideal fish to add to your 55 gallon that get along with your tiger barbs and platies are
* zebra danios
* cherry barbs
* cory catfish
* purple cichlids
* black mollies
* sword tails
and few other fish of your choice. You should multiply the length and width of the tank in inches. Divide that by 12. The number you get is how many inches of fish you can have.
5 or 6 yoyo loaches(Botia almorhae) or 4 or 5 Botia striata, any kind of SMALL loach would love that size tank. They will eat snails, clean up left over food and are so non-aggressive toward their other tank mates it's unbelieveable. they wrestle around with each other and are just down right personable.
Rams are harder to find at fish stores but they're not very big and yet they're beautiful. They would be great for a 55 gallon peaceful community tank. I'd also add harlequin rasboras for consideration. I always suggest for people's community tanks to have zebra danios, cherry barbs, harlequin rasboras, and some larger fish like rams (or even a bit larger), depending on the tank size. But, these are my personal likes