query different agents from same firm

It really depends on the agency. Some agencies have pitch meetings, where potential projects are shared with all of the other agents and group decisions are made. Some let their agents pick and choose their own clients. I do know that some agents state that if they feel it's a better fit for another agent at their agency, they'll pass it along to them.

I suppose that unless the agency guidelines specifically state to not query multiple agents, it's okay to do so after a rejection if more than one agent handles your genre.

What Chaos said.

Yep. For example: Andrea Brown Literary - only query one person per project. Writers House - query however many you want, but one at a time.

Yeah, query away, but not at the same time. And, I think some of the agents in NY might be shared, etc. So, it might be best to limit yourself in WH.