Will the 10G XG-SFP-SR-MM850 SFP+ auto-negotiate down to 1G if used in an SFP 1G switch port?

No, the 10G XG-SFP-SR-MM850 module is an SFP+ transceiver, which typically does not auto-negotiate down to 1G if used in a 1G SFP port.
SFP+ modules are designed for 10G and, in most cases, are not backward-compatible with 1G SFP ports.
The physical and protocol differences between SFP and SFP+ transceivers usually prevent this kind of compatibility.