Pixies only eat live foods, such a large aqaurium for them would present a problem with feeding density.
The 50 gal. would be excelent for a starter tank and Mustangs(H. erectus). You could keep a pair or two very nicely in that tank. The size also allows for better water. In smaller tanks the water tends to degrade faster.
My advise would be to set up a small tank for the pixies and a large tank for the erectus. I’m not saying keeping the two can’t be done, but I think you’ll be happier keeping them apart till you’ve got more experience.