Okay. . . so. . .Lets take a look at who is living here, exactly - without proper names, much of this is guesswork, so please correct me if I'm wrong. You can click on the shaded names to be take to the species profile for the fish highlighted to get more information. . .
Sucker Fish: What most people refer to as a Suckerfish is a Common Pleco. These beasts really have no place in an aquarium - they belong in a pond. They are very dirty animals, with a high level of waste. People are often told that they need them to 'clean' the tank, but in truth, they really make a tank far more dirty than it would have been without them. They're catfish, so don't like salt. Grow to be 18 inches or so, and can be very territorial toward others of their own kind. These shouldn't be kept in anything smaller than a 55g tank, though they are fairly adaptable when it comes to temps and water hardness - provided it's fresh.
Kissing Gourami: Lovely fish, when kept properly - and very easy for a beginner to keep. These guys aren't very fussy at all, as long as they are given clean FRESH water - and a REALLY big tank! Kissing Gourami grow to be about 12" each, and are aggressive. They will eat any fish that are small enough to fit into their mouths. NOT a good choice for a community tank. They'll need at least a 55g, but truly should be kept in a larger tank as they grow. Good with different water hardness and normal tropical temps.
Flounder? I'm not sure entirely what it is you have here. The most common type that I've heard of in tanks is often called 'freshwater flounder' and is Achirus lineatus - I think? To the best of my
knowledge, these are actually brackish fish, and need full saltwater as they mature. They grow to be around 6 inches long, will eat fish smaller than they are, are nocturnal, and very difficult to feed - they won't eat flake. . . at least not at first. But do post up an image of what it is you have so to be sure...
Fiddler Crab: I've never looked into keeping these, so double check my info. But from what I've read, and the random setups I've come across - these guys are usually kept in such a way that they have an area of land to come up out of the water, and do best in species-only setups (I/E no fish). There are many different types that I've seen for sale, and I'm sure they each have their own 'rules,' but I believe they're also fairly fussy as far as how much salt is in their water, and also fairly sure that most, if not all, of these DO require brackish. These do not belong in a community tank. . .
Molly: Now THESE are a great starter fish, if you have hard water. They can go from fresh water to pure brackish - so you should be just FINE with your Mollies. . . the only thing is that they do have a high bio-load (meaning they poo a lot) and they're very active - so they need a tank bigger than 10g. If they're truly only 1/4 inch long - they'd be fine for now in the 10g - but not with everything else :)
Guppy: YAY GUPPIES! THESE are ANOTHER great starter fish that will live in perfect harmony with the Mollies - AND can live in a 10g tank with no issues! They're live-bearers, so babies will happen and they like hard water. Guppies can tolerate a bit of salt, but you'll not want to put them in a fully brackish setup.
Not sure what your black and yellow catfish is...
Fire eels are cool, but grow to be 20 inches + and will eat anything smaller. . . no salt. . .
and you've been told all about the Puffers already - these guys NEED brackish, and do best in species-only setups. . .