1. No, there's no reason you can't have both MySQL and Postgres on the same machine.
2. Sort of--the default configuration of IMP uses MySQL for contacts, prefs, and session management. This can be changed to use postgres if you want, but you would be on your own.
3. No, I wouldn't say that, but instead that a general RDBMS server is outside the intended scope of the SME system.
4. As with most other additions and modifications, I'd think this is accurate.
5. Not necessarily, but it's certainly possible.
I don't know anything about Zope, so I don't have much of an opinion. I don't see any need to install Postgres as part of SME, as it seems to be beyond the intended application.