You can try adding some yamato shrimps (about 10), but they have they might go after your riccia. And yes, you can 2 more otos. Otos will not eat the hairy algae though.
Your SAEs are not eating the algae for a few reasons. One, not all algae critters eat all kinds of algae. Two, SAEs get spoilt on fish food and stop eating alge after a while. To prevent that, don't feed your fishes too much, or feed more frequently, but smaller amounts. That way the SAEs and other fishes (if they eat algae) will feed on algae to satisfy their hunger.
For your choice of plants, you will have algae problems because they do not take a lot of nutrients from the water. Cut down liquid fertilisation for now and see what happens for a few weeks. Change water more frequently (once a week). Feed less. Don't add too any more fishes, unless they will eat the algae.
You can also consider planting some stemmed plants to suck nutrients from the water. Rotala indica aka Rotala rotundofolia is a small leafed, sunset/red coloured plant that I think will compliment your current plants. Surf www.tropica.com and www.dennerle.de plant catalogs for more plants.
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