What you’re really asking for is a Flutter developer who knows how to integrate GraphQL. There are a few GraphQL packages for Flutter.
As far as security, there is only @auth security through JWT available for Dgraph. There is nothing like Firestore Rules until Dgraph fixes the @auth limitations like the update-after problem.
Basically you have to write custom @lambda mutations for every mutation you have to ensure it is secure, while disabling all regular mutations. This would be the equivalent to using a custom Firestore Function for every create and update (and a lambda-hook for delete).
While there may be one or two Flutter programmers here, I think your best bet is to find a Flutter programmer who understands GraphQL to learn Dgraph.
Good luck!
J