You mentioned loaches - they are bottom dwellers, and will tend to eat from the substrate. Some will also swim at all levels and eat from the surface or from the water column as foods sink; it really depends on the individual habits of each fish. They're social and need to be kept in groups, so watch your stocking levels carefully and stick to the small loaches (Kuhlii, Angelicus, etc.) as 40g isn't very large.
A few things to note, however: Most loaches require excellent water conditions, and when you say "...picking up excess food, &c." you should give up on the et cetera part. There is no such thing as a fish which will eat waste, and even if there were, any fish that eats is a fish that poops, and therefore creates more waste. You shouldn't add any fish or other creature to your tank unless you genuinely like or are interested in them, rather than merely wish them to perform some duty. Also, if there's an excess of food left to gather in the substrate, then you're overfeeding.
I'm sorry if this comes across as rude; I don't mean to be. I have two Asian tanks myself and it's one of my favourite biotopes.