Which repo should be used to add packages to an installed 2008-spring-rc1? Are 2008.0 repos OK for 2008.1 releases?

I may be reinstalling Mandriva after my first attempt ended up a little weird.
( see this post if you want to read more about that: http://mandrivausers.org/index.php?showtopic=52424 )

I don't usually try beta software but as there is a release candidate I might go for
mandriva-linux-2008-spring-rc1-one-KDE-int-cdrom-i586.iso

I'm just not sure which repository (if any) can be used with 2008 rc1.

No, it isn't.
You must use the EasyURPMI link at the top of this page, and set your Mandriva version as "Cooker". You can later switch to 2008.1 repos (they will appear by the time 2008 Spring will be out offficially).
However, if you are new to Linux, I'd advise you staying away from Cooker.