Well, I can't advise you on this because I don't have experience with RED stuff.
What I can tell you is that for the off-line you would need two versions of the Prores Proxy files, one p50 (for normal speed) and the other conformed to p25 in Cinema tools.
For FC there is not relation at all between a p50 clip and the conformed to p25, and FC doesn't know if the files has been conformed or not. For FC are completely different files.
The p50 files will keep the relation with the RED files so they can be re-linked, but I understand that the p25 lose any relation with the original REDp50s tuff and can't be relinked.
This what happens with FC and any kind of footage that you import or L&T. I don't think with RED footage would be different.
If you slow down in FC, then you would have no problem to relink (no conforming and no p25 files to deal with), but you won't achieve the same quality slow down.
The workaround could be converting to Prores (Hq or 444) the stuff that you want to slow down, conform the files to p25, and make Proxis to off-line in FC. Then you will be able to relink to the high quality Prores.
This is just my think, but is possible there is a better workaround.
I don't know if Shane's tutorial deal with RED files:
http://library.creativecow.net/ross_shane/tapeless-workflow_fcp-7/
rafael
http://www.nagavideo.com