If I want to make a Slush Fighter OC., should I make a new page?
Yes, that's literally how simple it is.
As for connections to the main SI, you have to make it of your own version but not taking full credit of the original (obviously). As well as connections with other users' own series, those have to be asked of from them and if they agree in making a connection with your OC, then it's settled but try to avoid less complicancies between both continuities if the agreement has been settled.
As for build up, you can always make sure that your character is defined through his/her traits, his/her powers not being way too overpowered, but if you do want such, at least make sure he/she has flaws that don't make them completely "god-like" and if you're going to ask for a portrait of your OC, ask nicely of other users who can do it for you. A bad first impression can lead to a lot of really unnecessary and bad first times with the others and likely to yourself.
Just something to say to you. And welcome to the Fanon wiki!
Do you have to add, like the picture or something? Or can you describe his appearance with, just simple words?
^ You can do so either way.