Don't get 2 dwarf gouramis, whatever you do. In general, don't get more than one gourami of any sort in that small a tank. 2 is asking for trouble. Attacks, stress, disease, oh the misery.
Stick with one.
As for the scavenger: you could try a dwarf bristlenose pleco, they stay small and can be quite interesting if you get one wiht an albino gene (striking white spots and sometimes stripes on the tail). Excellent scavengers/algae eaters as well.
Otos are good choice too. But they are more finicky on water temp/quality.