Well, that's an interesting idea. 9_6
You should take some more time to make sure the ponies look show accurate, or at least similar in proportion. For example, the head sizes of all three ponies seem off. Derpy's hindlegs could be longer too, and you forgot her bubblewraptastic cutie mark. Her eyes seem a little too small too. Also, is it me or does Discord's neck seem a teeny bit too long? o_O
As I suggested with another group drawing. If you're drawing a bunch of ponies together, you might as well add in a background.
Well, all you have to do is practice.