I believe if you go with 5.1.3 it should work.5k should be the same or higher os version.or upgrade 5k to 5.2(1) first, same as you are trying to upgrade to on the FEXs.
To use netflow you need to have the supervisor engine V-10GE or a netflow services card, such as WS-F4531 or sup IV or V.I am sure you already know this.
