20 gallons(about 81 liters) seems on the small side for those fish. A tank upgrade sounds like like a good idea to me. As for the shark, it will probably get aggressive as time goes by in that tank,but there is also the possibility that because it is growing up with those fish it might not be as aggressive as it would be if you just introduced it at a larger size. Still, you should get a bigger tank. Yay!