I think you should consider a so called "algae eater" only if you enjoy them like you do your other fish. Algae eating should be a bonus of having the fish and not the main reason. Most common algae eaters, various plecos, actually compound the problem as they add such a large amount of waste to your tank that existing algae problems are just fed by having plecos. Most algae eaters eventually get lazy when they realise they can just eat pellets or it is simplay a part of them maturing that they reduce algae consumption.
An appropriate maintenance schedule and feeding regime can work wonders, as does lighting hours and plants to out compete the algae for nutrients. Also sponges with a bit of elbow grease applied are great.
I agree on the rainbow shark with Lupin. They are far too "energetic" for such a small tank. In such confines it will harass your mollies eventually.
So your choices are:
*Let the shark get bigger and see its true nature come out and kill your mollies and you will either euthanise or rehome it.
*Or rehome it now while it is still young and get something more appropriate for your tank such as dwarf loaches or corydoras. This way no stress or deaths occur and you get a tank you can enjoy rather than stressing over.