I can throw in some comments about Project Types - I don't find it entirely necessary to simplify to just one. The key impacts from eProject's functionality are essentially Time Tracking style, Custom Fields, Phasing and Auto Health. Most Customers I have worked with however are usually only impacted by Custom Fields and Phasing.
Scenario 1:
You have 2 departments using eProject - Finance and Engineering. All they track in eProject are tasks and issues, standard phases apply and the only custom field they have is "Department" with finance/engineering as options. They can share a single project type.
Scenario 2:
Same 2 departments as above but Finance has special phases specific to the finance world. They also want project information to include Revenue, Approved Budget, SOX compliance status and Project Rank Number which are all custom fields.
Engineering wants their phases to be related to a development cycle - they also want project information to include Phase Gate Number, Develpment Cycle and Customer Name as custom fields.
In Scenario 2 you would want two project types (one for each department) because they have substantially different tracking needs.
Lets see what others are doing!