Sparklers and croakers are in danger of becoming snacks even when quite nearly fully grown. Sparklers only reach about 1" and croakers only about 2" (not including any fins of course as males can actualy have quire impressive finnage). Also, I'd be very worried about conflicts with the barbs as they occupy the same area in the tank and aren't particularly large. Tiger barbs don't only bully things with long fins - they also go after small, vulnerable, fish or fish that get in their way. It's all about establishing who's boss rather than simply all-out aggression or territoriality but it does cause some serious problems.
In the case of dwarfs, it's not a good idea to mix them with anything that'll cause them stress. Unfortunately, tiger barbs fall into this category so I'd avoid the mix. Dwarf gouramies are extremely prone to disease and even just a little stress can trigger illness.
The only gouramies I'd keep with tiger barbs are the larger, upper-dwelling species with shorter fins such as three-spots, moonlights and kissers - the latter probably being the best option as they don't have the modified ventral 'feelers' that are so very prone to being nipped off. Having said that, none of these fish will work in your tank - the last two need somewhat more space as they grow quite large and three-spots won't do well in crowded conditions and are likely to act very aggressively.
Actualy, your tank is almost fully stocked anyway. Of course, I have no diea what plec you have though. It would be a good idea to take a picture and post in the catfish section for identification. Like Miss Dib Dabs said earlier, some plecs get rather large. Commons, for example, grow to 18". All plecs are also extremely messy, very high, waste producers.