GigabitEthernet SFP transceivers are supported in 1/10G SFP+ Nexus 5k slots. Please see the Nexus 5k Datasheet (see Table 2) for details.
No FastEthernet SFPs are listed in the Nexus 5k Datasheet (see Table 2); as such, you're out of luck. FastEthernet SFPs are not supported on the Nexus 5k; however, many FastEthernet switches have GigabitEthernet uplinks. If that FE switch has a GE uplink, then use that GE port on the FE switch.
The old GBIC transceiver format was pretty generic and you could ignore branding on them. SFPs are a different animal, vendors qualify individual part numbers for support; you can even see different transceiver part number support between different switches from the same vendor.