It can be between two OC's, Two cannon characters, or an OC and a cannon character.
NO SMALL STUPID PARAGRAPHS! "-she walked into the room- hello" This is certainly not allowed. When role playing with me you should have at least three sentences.
I guess you can comment on this journal entry and I'l give you further details.
Here is a My little pony role playing website. There you can rp as an official mlp character (ask for permission first) or make up a character! Become a member and tell your friends! http://mylittleponyrp.webs.com/