I am not sure how large all of your fish will be when full grown, although it seems that your tank is fairly heavily stocked to me. Definitely check out the maximum sizes of your fish now, before adding a potentially 6" long fish.
That being said, my SAE has been of tremendous benefit in wiping out/keeping down the algae in my tank. SAEs are very peaceful fish (mine thinks he's a swordtail now -- I always see him and the two swordtails together), so he shouldn't cause any trouble to the fish you already have. As he's grown, I have found that my SAE will consume more than just algae (flake, freeze dried/frozen shrimp, peas, etc.). But he still continues to nibble away at the algae all day.
SAEs will grow to about 6" in length, from the information I have read. This was a surprise to me, because my LFS, which is usually fairly reliable, said up to 4" -- a big difference to a person with a 20 gallon tank! What is more important is that he has grown
very quickly. He's gone from about 1.75" to about 3" in just over 3 months! So when you're looking at how many (if any) more fish will fit in your tank, keep this in mind.
Finally, make absolutely sure that the fish you are buying is a genuine SAE. Sometimes the term Siamese Algae Eater is mis-used when referring to Chinese Algae Eaters or other fish. Be sure to get a true SAE -- the black stripe should run the length of his body to the fork in his tail. The other fish sold as SAEs can get aggressive as they age, and that will be a problem with the peaceful fish you are currently keeping.
Two good websites about recognizing true SAEs are:
http/www.aquatic-gardeners.org/cyprinid.html
http/www.petresources.net/fish/article/sae.html
Good luck if you get one -- they really are great fish.
-- Pamela