Can I drop a foreign key contraint from a table participating in merge replication? If so, can i just drop it from the published and will this chnage be replicated to all subscribes?
There's a dedicated article on MSDN around Making Schema Changes on Publication Databases
It does obliquely discuss the dropping of constraints, here:
It is recommended that constraints be explicitly named. If a constraint is not explicitly named, SQL Server generates a name for the constraint, and these names will be different on the Publisher and each Subscriber. This can cause issues during the replication of schema changes. For example, if you drop a column at the Publisher and a dependent constraint is dropped, replication will attempt to drop the constraint at the Subscriber. The drop at the Subscriber will fail because the name of the constraint is different. If synchronization fails because of a constraint naming issue, manually drop the constraint at the Subscriber and then rerun the Merge Agent.