Welcome to TFF.
To your question...solely from the perspective of fish mass to water volume, either otos or kuhlii loaches will work. But there are many other factors in deciding which fish for a tank, and as you admit to being new to this, I will try to explain it without too much data.
Fish size (mass) is obviously very important, but one also has to consider the species. Many are shoaling (sometimes called schooling though technically this is more marine fish than freshwater fish behaviours) which means they need a group; the number varies with the species. There can be several reasons for fish shoaling, and the first is usually safety--safety in numbers. Too few and the fish will be under continual stress, which weakens them and inevitably causes disease problems they would otherwise fight off, and a shorter than normal lifespan. Some species develop a hierarchy within the group; sometimes this seems to be primarily "play" (though I always think that to the fish it is probably much more serious than what we think of as play) but some species have a determined aggressive aspect to this hierarchy, and sufficient numbers usually mean no one fish is hounded literally to death. Studies have now proven that when the group of fish in a specific species is too small, aggression increases (for naturally semi-aggressive species) or appears (in otherwise peaceful species) and can be very pronounced.
Water parameters are important too. Freshwater fish have evolved for specific habitats, and their internal physiology will only function at its best when the elements of the environment that the fish "expects" are provided. Water hardness is key here, with pH also important, and temperature. The internal metabolism of fish is directly governed by temperature, with greater consequences for fish than for cold blooded land animals. The aquascape enters this, as it is a major part of the environment in which the fish is designed/intended to function.
The other species in the aquarium are also important. Fish interact, within the species and from species to species. They send out chemical signals, pheromones to their own species, allomones to other species. Physical aggression is not necessary to cause stress to a certain species if another non-compatible species is present.
The activity level of a species has to be considered; putting active swimmers in with sedate fish is not going to work. Similarly, water current from the filter can impact fish if it is too much or too little depending upon the species as they have different needs.
So, with all that in mind, it would be useful to know your source water parameters, thinking hardness (GH) and pH. The KH is also worth knowing for reasons I won't bog this down with just now. You should be able to find these numbers on your municipal water authority's website. It is much simpler and safer to select fish that will thrive, relatively speaking, in your water rather than attempting to adjust water parameters for fish requiring something different; this can be done, but it is not usually easy and can create considerable problems.
Second, it would help to know if you intend more fish than just the kuhlii or otos. Both of these are somewhat "unseen" in aquaria; I have otos in one of my eight tanks and often have to look carefully to see them. Kuhlii loaches are well know for burying themselves in the substrate (sand is essential for these fish for this reason), and getting into the smallest hiding spots; they are somewhat nocturnal. My thinking here is that you will likely want other fish to add some interest. And both species are shoaling, so a smallish group is needed; I would suggest no fewer than five of either species should work fine.
This may get thing started.
Byron.
