I agree with scottychaos, that tank is pretty small and I would avoid including anymore fish. If algae is your problem, I'd consider doing more water changes (you should be changing at least half the water once a week for this small tank IMO), and scrub the walls and so forth to remove algae. Algae is many times a sign that there are too many nutrients in the tank, water changes are important to reduce their levels...is your tank cycled? Ammonia especially is great for growing algae.
Also, reduce the amount of light exposure your tanks gets: reduce the overhead light on the tank (get a timer, 8 hours is more than enough), and get it away from any sunlight.
IME, algae eating fish like otocinclus also have a tendancy to produce a lot of waste, and produce more waste than other fish their size. They can dirty up a tank pretty well. If I had to choose an algae eater for your tank, I would go with some tiny snails, like ramhorns or malaysian trumpet snails (MTS).
I hope this helps.