if the angel is still small adding more neons and black neons would be alright. but once they get big enough to eat them if you add new ones that they dont recognise they will eat them. if they grow up with the neons or other otherwise small edible sized fish, they recognise the individuals and wont eat them, but once they are big enough to eat them (usually by 2" body diameter) you may have trouble adding more. and no, you dont really have room for medium fish. what filtration do you have? what is your water change schedule? both of these issues have to be considered when determining stocking levels. you cant stock based just on tank size.