Sounds like a colorful, interesting tank. They MAY get along....if you have at least 6 of the tiger barbs....but I also worry that they will just end up as a tasty morsel for the convict.
The biggest problem I see, is that in a 30 gallon tank you would be grossly over stocked. If you only had 6 TB,,, it would be at least 34" of fish. That is counting the low estimates on the fish. This doesn't sound so bad, but with several large waste producers, it would not be easy to keep up with the tank maintenance.....
The other thing I see is that you are suggesting a Clown Pleco. You would NEVER see this fish in an aggressive tank, and it wouldn't be a good addition. It would get picked on all the time.
You could lighten the bio-load by not using this fish in this aquarium.