Are 5 minnows and 2 shrimp ok in a 20l tank?

IMO too small a tank. 20liters= 4 UK gallons or 5 US gallons. A few small peaceful shrimp like Amano, Cherry or Ghost shrimp (as well as critters like Apple Snails & African Dwarf Frogs) would be totally fine, but i would avoid any shoaling fish (even small ones) as the tank is too small and so the fish may show abnormal behavioral problems- good water quality would also be difficult to manage in the tank. Personally i would just keep a lone male Siamese Fighting Fish/Betta in a tank that size.