Is it against the rules to advertise for an RP that is happening on another service like KIK?
Not at all! 
You're more than welcome to post a topic in the Looking for RP forum/the RP Finder and take your RP to KIK, Skype or anywhere else you like.
You're more than welcome to post a topic in the Looking for RP forum/the RP Finder and take your RP to KIK, Skype or anywhere else you like.
Go for it.