My initial thought is that you can't do this (i.e. the database having a configuration for Streams and Data Guard).
I may be wrong but I do not see a reason why you cannot use DataGuard physical standby of DB that has Streams replication.
However if you use Async transport there can be risk of loosing last Streams transactions, I do not know how Streams will tolerate this. I think rebuild/reinit of it will be required with resynchronization of all target tables.
If you use Sync transport this will not be an issue, but performance may suffer.