Welcome to Tropical Fish Keeping forum. Nice to have you with us.
I agree with what has beeen suggested re fish, that in your 5g you can have a single Betta but no other fish with it; or a group of 7-8 of one of the "dwarf" species that redchigh mentioned, subject to your water parameters. You are correct on the combo issue, I don't think redchigh meant guppies/Betta together but "either or."
Your gravel is fine, very good size in fact, and a nice shade, darkish. Excellent.
The Anubias is vulnerable to brush algae, due to its low light requirement, thick leaves, and being very slow growing. Otos will not eat this algae anyway, nothing will in fact (except a fish that is way too big for your 5g so I won't even mention it
). You might want to limit/reduce the light over the tank. If you tell me what it is, I can suggest something. Daylight in the room also has to be considered.
Anubias is very low light requiring. One good idea is to have floating plants. Especially good with the fish we have been mentioning, plus floating plants reduce the light and Anubias love growing in the shade provided by floating plants, and floaters filter the water well because they are fast growing.
Anubias will do well attached to rock or wood, as long as the rhizome is not buried as someone mentioned.