AFAIK a SFP+ (read: SFP Plus) Transceiver goes at 10 Gbps Full Duplex, not any lower speed like 1 Gbps (a SFP Transceiver - withouth the Plus suffix - works at 1 Gbps Full Duplex), so you can't down rate a 10 Gbps SFP+ Transceiver's port speed as you would do on a typical RJ45 10/100/1000 Mbps copper port (the SFP/SFP+ Transceiver sets the speed and the Switch slot in which that Transceiver is inserted lets it to communicate with other switched ports, you can't change the Full/Half Duplex Mode or the Speed).
So if you're using a SFP+ Transceivers pair the fiber optical link should be up at 10 Gbps Full Duplex (provided that the Fiber Optic link is clean OK).
The only way to go at 1 Gbps Full Duplex is not using a SFP+ Transceiver but using, instead, a SFP one.
Which SFP+ Transceiver(s) and which Switch(es) are you working on exactly (HPE/Aruba SKUs would help)?