I have to move some projects to svn 1.7, therefore I am forced to use subclipse. Can I define on project level if subversive or subversive shall be used? Or is this only definable if I create a new "checkout from svn" project?
Yes you can, but the problem is that it is virtually impossible to tell Subclipse and Subversive apart in the UI. So when you use Team > Share to connect a project you will just see two SVN entries in the UI.
The best thing would be to just pick one of them and use it for all.