Last year I took in a flying squirrel and sugar glider couple. They got along OK. The sugar glider was very protective of the squirrel. But, neither of them looked very good from grooming each other and the squirrel looked TERRIBLE from being marked by the sugar glider.
Diet is particularly difficult when housing them together.
Eventually, Leche (the squirrel) bit Dulce's tail and Dulce had to have a tail amputation. His healing time was the perfect time to separate them, and they are both much healthier now.
They sure were cute together, though! Here's my cute couple: