MicrostockGroup

If you have experience, please share, how are agencies accepting these character sets made of leaves, flowers, colorful beads, etc...
I know only about SS and DT. As I saw, SS accepts standalone characters, and DT likes only groups (based on some forum discussion, years ago), however, I browsed the newest images @DT and I saw standalone characters.
How complex must be one letter to go standalone?
It is better to group 2-3 characters together?
What are the rules at the agencies (= how they accept characters? (Standalone, in 3-6pcs/image or whole set)

Thank you!