Symposium does not act as a core replacement, rather integrates within it; as such, third party plugins should be fully compatible.
With "Login as", AFAIK, you can log in any user but can't see the user PMs for privacy and ethical reasons. The same ethics, of course, apply to Symposium, and I cannot make any exception for that.
"Login as" permit admins to view any member's pm without symposium.
It's sure that it is not very ethical but I do not consider my forum as a good place for private conversation (there is much better encrypted service for this). It's just that sometimes when there's a conflict between members it is a really helpfull tool to see what they told each other (scammers hunting).
If it’s allowed by Login as, Symposium should be compatible.
My bad it comes from newer version of Login as, I did use it before and wrongly thought it was symposium blocking this (did not want to deactivate to try on live board :/) sorry for the mistake.