IMO you can easily do 5 frogs in there. In a species only tank I stock 1 per gallon and they really thrive. Their bodies stay about an inch long w/ a relatively small bio-load, and they are aren't incredibly active. They enjoy eachothers companies, and IME act the most "frog-like" in larger groups.
For the gravel, I would use smallish smooth stones. Colored type gravel can be too sharp, and larger smooth stones can make it difficult to keep decor safely planted in.
You mentioned algae, I keep a nerite snail with my frogs and it keeps everything spotless, I would pick one of those up as well if you can. Just don't get a horned variety so they don't hurt themselves on it. They'll enjoy the anarcharis. I would also pick some plants with broader leaves so they can sit in them..even just simple java fern works great for them. I keep a couple types of nice leafy swords with mine too that they enjoy. The really like live plants. I kept them for years with fake plants (what was I thinking lol) and I hardly ever saw them in a plant..now they are in them all the time.
I would invest in a good heater if you can. The first one I used for them had a preset temp and it wouldn't get the water quite warm enough. They do great at 80F if you can get it there. 78 would be OK too though.