I am a new user to Devonthink Pro Office as well as Devonthink togo…
I only use Dropbox for syncing purposes.
When using dropbox to sync, can I use selective sync and NOT store the devonthink sync file locally? Or does it need to sync to my laptop?
First, welcome. 
Secondly, I hope you don’t have your database in your Dropbox folder. If so, you need to quit DEVONthink and move the database. We strongly suggest a folder in your Home Directory, like ~/Databases.
Thirdly, you shouldn’t have any Sync data in the Dropbox/Apps folder on your local machine. In the Dropbox application’s Preferences > Account > Selective Sync, make sure you uncheck the Apps/DEVONthink Packet Sync folder.