I would go with either:
A betta (male) and 2 nerites or 1 nerite and mystery snail and if you go with sand you could fit some very small corydoras. Dainty corydoras or Pygmy corydoras. A group of 6 would work well.
7 White clouds and the snails mentioned earlier
I wouldn't mix white clouds and betta. Although I have done this in the past and yes it did work, white clouds are like gold fish in the fact they like cooler water and are "subtropical" (76f or lower) and bettas like warmer water. White clouds also breed like crazy
Another option is rasboras. Hengeli or Espei rasboras are super cool and are a little smaller than harlequin rasboras. They like to be in a group of 5 or more. Again - a nerite or mystery snail is tolerated with them
I wouldn't suggest celestial pearl danios only because A. they're REALLY shy and I've heard they're a bit sensitive'
I have 2 female dwarf sunset gourami in a 46 gallon. They surprizingly get along well, but you could do 1 male or 1 female instead of a betta