I edit ProRes in Premiere all the time, since up to this point I have still been doing my grading in Color. If you have the ProRes codecs installed on your machine (which you should, given that you're running FCP7), Premiere will edit them.
Premiere will also handle DNxHD in a Quicktime wrapper. You can get the codec from Avid as a free download:
http://avid.force.com/pkb/articles/en_US/Compatibility/en350875 or by installing a trial version of Media Composer.
I've found that using an intermediate codec like ProRes is less of a hog on your system resources than many native codecs, therefore it will give you snappier real time playback. Especially once you start mixing camera formats and adding effects.
And to correct Alex: Traditionally Avid relied entirely on its own internal media management. Now you have the option of using the Avid Media Access (AMA) feature to directly link media on your disk to your bins, with or without transcoding, just like FCP and PPro and all the rest. From what I've seen, AMA works pretty well, but I've got limited experience with MC so I can't really speak too definitively on the subject.
