Well I usually create the OC when I get inspiration for what kind of character might have an interesting interaction in the RP. As of right now, I don't have any, but if I come up with an idea, I'll put it down here.
In general, I love roleplaying as some of the Mane Six characters. I think that some of them could have a lot of potential in this RP. IMO, I think that Rarity would be most likely to agree with Pristine Polish, given how she views being dirty and messy as being uncouth. Twilight Sparkle probably would get along well with her as well, preferring to stay very well organized. What I think could have an interesting conflict would be Applejack, given how they both value working hard. However, I feel like Applejack would view getting dirty as an essential part of hard work, given how she works on a farm. Finally, I think that Rainbow Dash and Pinkie Pie would have the most conflict with Pristine Polish. Both Rainbow Dash and Pinkie Pie are messy and chaotic. They would probably want to prank Pristine Polish. Maybe drag her into a paintball fight or food fight for Rainbow Dash and Pinkie Pie, respectively.
Hmm, Pinkie Pie being suspicious of Pristine Polish? That could be quite interesting...