I'm interested in turning my 10 gallon brackish because I have mollies and one guppy. However, I also have two african dwarf frogs. Can they tolerate brackish water?
In a word, no. There are very few brackish water amphibians and only one marine species (Rana cancrivora). Your African frogs will not appreciate brackish water and will likely be harmed by it in the long term.
There *are* scientific experiments where Xenopus laevis were kept in brackish water for extended periods, and at low salinities (25% seawater or less) the frogs came to no harm. But the problem is that lab work is based upon keeping animals for days or weeks, not years, so it is difficult to know whether such experiments mean the frog is genuinely salt-tolerant or merely able to put up with poor conditions in the short term.
Also, which algae eaters stay small and/or can tolerate brackish water?
Your best bet for a small aquarium are Nerite snails, of which there are many varieties, some sold in fish shops, others available mail order. Olive nerites would be ideal. They eat algae and not much else, are very hardy, and rather pretty. They do not breed easily, so you won't get thousands of them! At low salinity (say, SG 1.003-1.005) you might also try Colombian ramshorn snails, but these eat plants as well as algae. Amano shrimps should also do well, if you wanted something a little more active.
Cheers, Neale