JI am wondering about the power thing as well, how often and for how long does it go off? I am sure you know that having constant fluctuating temperatures and having the filter not work for a period of time is not good for the bacteria or the fish.
Anyways, I feel like your selection of fish is wrong for what you are going for. You want fish that are outgoing, colorful, and hardy; while rummynose tetras, glass catfish, and otos, really aren't. Rummy nose tetras aren't the hardiest of fish, and are kinda pricy in my opinion. Glass catfish are very expensive (or at least here they are, at least $5-$15 each depending on if you go to petsmart of an actual fish store) and they are quite shy little guys, and you need a school of at least 5. Otos are very fragile, and you either need abundant amounts of algae, or you have to feed them fresh veggies, and they need water that has very high o2 levels, such as a planted tank with a spray bar or 2.
Aquatic frogs are awesome, I love DAF!, but they are a pain in the butt to feed. You have to use a bastor or tweezers or something to make sure they get food, and they need food like frozen bloodworms/shrimp, they can't survive on just flakes that drift to the bottom or something. Definitely not low maintenance.
As for kuhlis, they are fairly low maintenance, I feed mine shrimp pellets and frozen food, but I hardly ever see them about unless it's dinner time or after a water change or something. For the most part, they just hang out in the driftwood, and poke their little heads out at me. Also, they are better in groups of at least 5, three is not enough for them to be happy.
As for shrimp, I've only had hitch hiked ghost shrimp, an a hitch hiked cherry shrimp, both which my betta decided to eat of course, so I don't have much to say for shrimp.
As for plants, what I would do is do what I have right now in my 29 gallon aquarium, I have 5 pieces of Malaysian driftwood and anubias nana (both nana and golden) attached on them. It makes for a pretty tank, my betta has tons of hang out spots, and if I need to move stuff around/out of the tank for any reason, all the plants are attached to the driftwood, so it makes it super easy. I just have the light that came with the hood when I got it from petco, and the anubias have been thriving, although the javas aren't (I believe the water is too soft maybe for them?). Even without fertilizers, the anubias are thriving. I bought the anubias nana and anubias golden on ebay for super cheap, I got like 20 plants, some were small others large, but they grow fairly quick.
As for substrate, I really, really, love this stuff, money well spent! Check out Carib Sea Instant Aquarium Substrates on Dr Foster and Smiths, I'd post a link, but for the life of me I can't go to their site, it keeps saying internal error. Grr. For a 20 gallon long, all you would need is 1 20lb bag, which runs $15-$20 on there, and you also can get some cheap Malaysian driftwood too.
As for the filter, there really is no room for a hang on? My aquarium is only 4" away from the wall for my HOB filter. To save money, I would purchase a cheap whisper or marineland HOB power filter from walmart or something, but then buy aqua clear foam on amazon and trash the cartridges the power filter came with.
Some fish I would consider for your 20 gallon long
-corydoras catfish (5 or 6, 6 if you go with panda, 5 if you go with pretty much any other species, don't get peppered or emerald~which aren't even cories because they get to be 3"+) Just make sure you get shrimp pellets as well to feed them, although they do eat spare flakes and pellets like and love to vacuum up bloodworms, it's adorable!
-male platies (4) They are very colorful, and with a group of 4 then can establish a hierarchy, just don't get females because then you will end up with far too many babies (unless you want that, and will find good homes for the babies, then make sure you have 3 females and 1 male)
OR
-male guppies (4 or 5) They are very colorful, and very pretty fish. Most people know what a guppy is. Get all males to avoid babies, or get 3-4 females and 1 male)
(If you get both genders of the livebearers, then I would leave the stock at that so you have room for babies, if you just do males, then you could add 2 or 3 more males, and leave the stock at that, it would make for a very colorful tank to have the cories on bottom, then having 8 male guppies or 6 male platies on top)
If you want to go more of a soft water route, then some good fish would be for a calm tank
-Glolight or black neon tetras (6 or 7) They are very pretty, and seem to be fairly hardy tetras in my opinion.
-Dwarf gourami (Although I have never had them myself, I do know it's a hit or miss thing for them and they can get bacterial infections pretty easily, so make sure that you get one from a very good fish store)
-Betta, male or female (don't get with the gourami, but I know at most fish stores I go to now they have bettas with their other fish, get one that gets along with other fish, I am always biased to bettas, I absolutely adore them and hate they are kept in tiny cups)
Some other fish to consider
-Danios (I personally love danios, especially the long finned leopard ones, get a school of 6 or 7, but since these are cool water fish, make sure you get other fish with similar temperature requirements.)
-White cloud minnows (Same as danios)
-Cherry barbs (a school of 5 or 6, with 1 male and 4 females, or 2 males and 4 females, good midwater fish)
As for finding cheap/free stuff check out
-Craigslist, both the PETS section and the GENERAL section, search "fish, tank, tanks, aquarium, aquariums, heater, heaters, filter, filters, driftwood," etc, CL is very finicky with wording, so search different terms, I can't tell you how many times just by adding an "s" that my results doubled
-Freecycle (yahoo group, join the one for your city)
-ARC, The Good Will, any thrift stores
-Garage Sales
-Newspaper
Hopefully this helps you! I wish you luck!