JorgeA,
1. See the following thread for discussions of Windows 7 compatibility:
Test Windows 7 compatibility
2. You should be able to use Spybot S&D as an on demand scanner without incident. I know of no specific problems using Teatimer with MSE, but I have not tested it as I avoid using them together on a daily basis. I use Spybot S&D as a tool and for systems which don't support MSE, so I would use Spybot S&D to attempt removal on any infected system no matter what else was installed.
3. This is really a question for the MSE forums, but to reduce confusion I'll answer here. Windows Defender still ships installed on Windows 7 as it did with Vista. MSE should disable Defender when it is installed since it includes the same antispyware database, so it replaces that function.
MSE will not be shipped installed with Windows 7, it is a separate download as Defender was with Windows XP.
Bitman