I'm not an expert mind you, but have been doing freshwater tanks for nearly 3 years now. I've got a 40g long with ~23 fish of various rasbora and tetra species, as well as around 12 cories of mixed species, and it's been very stable this year.

So comparatively in your 20g, 5 danios and 5 cories would be like 1/4 of the bio load in 1/2 the tank space, so I personally think adding an algae eater and clown loach would be totally fine. Granted there are multiple types of algae eater (some get pretty big!) and obviously the loach is a growing boi, so I can't be like 100% "Yes definitely this will totally work" but as long as you keep an eye on it, water change/test water quality regularly, it should be good. As always, there are so many variables in fishkeeping, even just with the water itself, so nothing's guaranteed.

TL;DR: In my experience, seeing as you could probably house 10 danios and 10 cories in that tank with no problem (depending on filtration setup and frequency of water changes) I feel like 5 danios and 5 cories are only about half the load for your tank and adding the algae eater and loach won't overload it.