Just to add to what Annastasia said, most livebearers don't like to be kept alone either. If you want something small enough to be kept in a 7 gallon but in a group, endler's livebearers or small guppies would work. Stick with 4 or more males in both cases so any aggression is spread out 9endler's are better about getting along than guppies). I'd also suggest heavily planting the tank to make everyone feel more comfortable. I don't think ADFs eat shrimp but ask in the amphibians + inverts section about that specificaly and I think there's also a pinned topic on caring for ADFs you may be interested in.