that 7 neon tets, 6 tiger barbs and a common plecostomus is overstock for a 20 gallon??????im not planning to do that but someone already has and is having major test problems after he cycled it....
It can be done if he/she is planning on using the 20g as a grow-out tank and if the tank has been previously cycled, but yes it "is" too many if the tank is not cycled yet - and the person is putting his fish at risk with such a heavy bioload right from the get-go.
Barbs don't kill neons unless there is alot of barbs and one neon,how big is the plec,you can stun the growth but not recommended,maybe exchange its place with a bristlenose but a bristlenose will prefer plants.
Tiger barbs probably wouldnt kill neons but theyd most likely chase them around and harass them. Better choice for tank mates for neons are fish like black phantom tetras, white clouds, harlequin rasboras and other small fish.