A 15L is tiny and going to require incredibly frequent maintenance to keep it looking nice and healthy. There aren't a lot of options for tanks this small. Like you said it's largly tiny corals and a couple small Crustaceans and a snail or two. If you really only want a hermit crab or two it will work out - but is that really what you want animal-wise? I have kept hermit tanks like that, but hermits are one of my main points of interest in the hobby so I'm a bit of an oddball for doing that. If you love hermits and tiny shrimp then great - there are some interesting-looking ones out there. There are also some beautiful predatory snails that could be housed in a species tank that size with just a bit of live rock. Similarly you can do some of the smaller mantis species like Neogonodactylus in tanks that size as a species tank (but with an acrylic tank! Wouldn't do glass because they tend to be super-thin at that size). But, the risk is that if you get bored with whatever tiny critters you put in there, you will be temped to overstock. Paritcularly if you are a fish person rather than an invert person, then you need to be looking at bigger tanks.