The cheerful Gang really doesn't fit into the picture. The sharpteeth and Tarzan make a good composition on their own, though. The only thing missing is something that distracted the sharpteeth from attacking Tarzan.
I also must commend you for the patient image cropping. This is one of the better juxtapositions I saw recently.
Well, this is how I make my fan art since I am a terrible drawer. I just take clipart from the internet and arrange it nicely on a background using Photoshop. Although I am planning on writing a fanfiction about my LBT Tarzan idea.
P.S My LBT Tarzan idea is what made me join this forum in the first place. I am really interested in the idea of humans in The Land Before Time.