If black-tip you mean, the Columbian or silver-tipped shark they get about a foot, but buy then they should be in a saltwater or atleast a brackish water tank. The pictus should get around a foot also, so you are really looking at up-grading your tank to around a 75-90g atleast!!!
you shouldn't have too much problem with either fish as both stay small. Your shark maybe a foot, but pictus much smaller. You will find that your shark will do better in brackish water especially after he starts to grow some more. I would seperate them into seperate tanks!