On the federal side at my own agency, I served hundreds of warrants...many as an SRT member for a period of some years, but most with members of the group or task force I was a member of at the time.
In the Florida Joint Task Force, (DEA & U.S. Customs), I was often the go-to guy for planning an op due to my SRT experience, even following my departure from the team.
At HIDTA, we usually handled warrants with our group members which comprised federal and local police and sheriff's deputies.
Don't remember a protocol being in place early on...it was fairly informal in that the group supervisor would reach out to the SRT leader and request support for an op. Later on I believe the practice became more formalized and SRT was requested more regularly for high risk warrants.
We had a relationship with other teams in South FL...Miami, Miami-Dade, (formerly Metro-Dade), Homestead, BSO etc and could call upon them or their bomb squads as required.