I don’t really have guidelines but I posted this a few days ago:
http://kpopfantasyworld.tumblr.com/post/34495109442/important-notice
Just read through it :)
I’d say, request one at a time because I get overwhelmed and I don’t want to close the request box for at least a while but if I get too many then I’ll close it till I finish all the requests

[gif not mine]