I am using Mandriva 2007, is it possible to use RPM's and stuff from other distro's say a fedoracore rpm perfectly fine, or are there just a few errors when you do it?

Different rpm based distros may have a slightly different filesystem tree (where files will be placed) and could cause problems when files can't be found where they should be located (according to your distro).

The safest procedure would be to get the .src.rpm (from any rpm based distro) and use it to make your own binary installable rpm for your system. Then install the binary rpm you just made. The compilation will be made according to the rpm setup of your distro, and the files will be installed where they should be, according to your distro's setup.

i actually don't think using the source rpm is any better. the source is setup to compile for one distribution alone.
when you build linux you are faced with lots and lots of choices about configuration.
as you go along sometimes you encounter bugs that need fixing and what you end up with is a rather unique creation we call distribution. they are not compatable with each other at all.

with that said sometimes with testing a package from another distribution is just what you need.
but as a general rule no way.