Yes, Brahma and Shiva have been granted the status of Srivaishnava, as per Srimad Bhagavata Puranam. Svami Desikan also mentions them as primary among the Sirvaishnava-s.
As asked, one can recite any stotra-s if it is available as a servant of a servant; there are some such mantras found in the Vedas; the Purana-s have some stuti-s. We do recite such mantras and stuti-s. But we must not consciously do so as a servant of servant and worship them at temples. These practices have been put in place as protective measures – that we must not be misled from the fact that Narayanan alone is the Supreme Truth; there must not be any compromise in this. It is enough that we follow the recommendations from Itihasa-s and Purana-s and nothing additional needs to be attempted.
As per the Purana-s, both Brahma and Shiva do reveal that Narayanan alone is the Supreme Truth (Paratattva); they will reveal this truth after a while and not at once. Purana-s say that after 7 or 8 births or several births, they will unite their followers with Emperuman. Many Purana-s have mentioned this – one who has been a devotee of Rudran for many births transforms into a devotee of Perumal. These depend on one’s dedication. Because certain prescribed methods are to be followed, they will not reveal this truth at once and will do so after a while.