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! 
