Do I need all versions of Logic
I have upgraded to Logic 9, but my applications menu now lists Logic and Logic 9.0.2 as well as 2 Logic Node icons, 2 Mainstage, etc. All of this has happened since I've upgraded.
Can I trash the older versions?
the app listed as Logic.app is the latest version, by right clicking on the name and selecting get info you can verify this under version in the get info window that will appear.
LP 9.1.6 is a 64bit app whilst 9.0.2 is not, the same applies to the versions of Mainstage you have.
also you could check this document : Logic Pro and Mainstage 64-bit FAQ