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This was an old 5G AIO that I got from Petsmart, and I used to keep two Scarlet Badis, a school of Kubotai Rasboras, some Tangerine Tiger Caridinas (shrimp), some Blue Dream Neocaridinas (shrimp), a pair of licorice gouramis, and a Bumblebee Otocinclus. There's a complete world of nano fish that's often overlooked, and it's entirely possible to make an awesome shrimpless nano fish tank. That being said, I am lucky enough to be somewhat close to a good aquatic expo that takes place every now and then, and a real nice LFS that carries a lot of oddballs. Either way, for stocking 5G tanks, my list generally composes of: smaller Badis species, Licorice and Sparkling Gouramis, Any Betta species (I know you said that bettas make you feel rather sad, and I can personally understand a part of that, but there are so many unique and cool betta species out there that don't resemble selectively-bred Splendens), killifish or most Fundulopanchax, although a 5G would be pretty small for one, Kubotai Rasboras, Celestial Pearl Danios (CPDs), Chili Rasboras, Gold Tetras, Harlequin Rasboras, Neon tetras (green, black, gold/platinum, albino, etc), Ember Tetras, White Cloud Mountain Minnows, African Butterfly Barbs, and the like; Dwarf Anchor Catfish, most Kuhli Loach species, Dwarf Habrosus/Chain Corydoras, Pygmy Corydoras, small halfbeaks (rather sensitive, rare, and oftentimes brackish, however), Endler guppies, various Otocinclus species, African Dwarf Frogs (not fish, but still), Crocodile Toothpick Fish (difficult to care for; select micropredator), Freshwater Pipefish (also select micropredator), Imitator Pea Puffers and regular Pea Puffers (both must be kept species-olIn terms of plants, just know how the specific species grows (epiphyte? rooted in substrate? column feeder? substrate feeder? ferts? CO2? light requirements?) and adjust from there; I would also recommend a good substrate (Fluval Stratum or Aquavitro Black Humate seem to work the best for me. Also, if you do choose to use these, DO NOT RINSE THEM, OR THEY WILL DISINTEGRATE), as this way you don't have to use root tabs as often. Good starter plants (in my opinion) are: Most Cryptocoryne, Most Vallisneria, Java Ferns, Anubias (although the variegated kinds often seem to melt without CO2 for me), some Bucephalandra, Amazon Swords, some Rotala species, Alternanthera, most mosses, Tiger Lotuses, and the like. All of these do best with a nutrient-dense substrate except for the Java Ferns, Anubias, and Bucephalandra. In terms of lighting, almost any bright LED will do, unless you're going for more high-tech plants. As for CO2, unfortunately I can't help you there, seeing as I prefer my low-tech setups to high-tech. In essence, a decent substrate, a decent light, basic knowledge of what you're doing, and that's pretty much it. As for smallest tank size, I'd personally not go anywhere smaller than 2.5 gallons, although tanks as small as a gallon can keep some very small species or fry. Would personally recommend a 5-7 gallon to hold a fair amount of fish. And yes, an elongated profile does help increase surface area and therefore can allow for slightly more fish, although there's a balance between far too long and long enough to be beneficial yet not detrimental- personally would say that if either the height or the width of the tank is shorter than 5 inches, then it's probably not as viable, although this is due to personal speculation and most likely is false. As for filters, sponge filters are generally pretty quiet and regarded as best, although most well-constructed filters with an outtake below the waterline are nearly silent as well.
Sorry if this isn't very helpful- I'm probably not one of the better advisors on this forum.
