-What is difference between a SFP and SFP+ Port?
SFP ports are 1 Gigabit Ethernet (1GE) and SFP+ ports are 10 Gigabit Ethernet (10GE)

On NSa 6600 and SuperMassive 9000 series appliances, X17 – X19 are 10GE (SFP+) ports. Ports X16, X18, and X19 are designated with a dot to signify that they are directly connected the CPU for maximum throughput. These dotted ports have a dedicated (non-shared) uplink to the CPU whereas port X17 share the CPU uplink with X0 – X15.
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 9×50) do support SFP modules in SFP+ ports. SonicOS will detect them and limit speed appropriately if it found SFP module in SFP+ port.