VSTS 2008 Database edition should be able to install on top of developer edition. I do not know why you did not get the database project templates in VS though. If you intend to use DB edition from VSTS2008, we can try to repro your problem locally.
No, you do not need to install power tool.
No, GDR R2 should just work on top of VSTS 2008. GDR R2 uses different project templates (not the same ones as in VSTS 2008). I believe GDR R2 installation laid down the project templates correctly for you.
Let us know if you see any other GDR R2 issues.
--Joyce