Typically business-critical and heavily multiplexed links (e.g. the upstream backbone connection or a number of downstream enterprise switches that each have 10Gb uplinks) should be connected to a dotted interface to ensure maximum performance.
Can a SFP+ optic plug into a SFP slot and will it function? Yes, a SFP+ optics can be physically plug into a SFP slot. SonicOS will treat the SFP+ optic as a 1GE SFP optic.
Can a SFP optic plug into a SFP+ slot and will it function? Newer appliances (NSA 3650-NSA6650, SM 9x50) do support SFP modules in SFP+ ports. SonicOS will detect them and limit speed appropriately if it found SFP module in SFP+ port.
In SonicOS for SFP 1G Optical module in SFP+ Ports, to make it compatible with Third Party switches, a user is allowed the ability to select between “AutoNeg” or “1G” speed selection to link UP properly (ensure the selection setting are the same at both ends.)
For the 1G Copper SFP module in SFP+ Ports, 1G speed option is only allowed due to Physical Link Limitations.