FPC7 is dead, unless you are planning on using it to convert your old archived projects to something useful in the future.
Everyone has either chosen avid, premiere pro, or decided they will keep playing with fcpx to see how it develops for their needs.
If you have fcp7 on an existing computer, I suggest you use the many tools available here:
http://www.apple.com/finalcutpro/resources/plugins.html
http://automaticduck.com/products/pefcp/index.html
http://assistedediting.intelligentassistance.com/7toX/about.html
SPECIFICACALLY:
Pro export fcp
7toX
etc.
To convert/re-archive your old projects/timelines to newer NLEs, so you can reuse them when necessary.
As far as the rMBP, if fcp7 even works on it, it will look ugly under the default "retina" display setting becuse fcp7 is not optimized for "retina". If you scale the display setting to "looks like 1920x1200", It should look fine. If it manages to run well, then you can do some fun playing around, but I would still recommend you spend your time learning/Working with newer NLEs.
I know I'll probably get flamed for this, but:
High end professional (feature films, tv shows, commercials, etc) should be on avid media composer
Mid-level prosumer/"professional" can be premiere pro (or maybe fcpx)
And anything else can be whatever you choose for your preference.
I've used all of these NLEs, and I personally recommend Avid. It has been the professional standard for a long time, has amazing backward compatibility, is dependable for working with other post-production facilities, and complies with silly things like quality control and broadcast standards in case you want to put your cool new movie on a real screen.
I suggest you wait for avid to release an officially supported configuration for the new rMBP before making your purchase. In the mean time you can try their free demo to get used to the ui and workflow.
Anyway, that's just my opinion.
Try them all and see which tool will lead you down the path you want to travel.