Backing plates for the cleats... yes. You can buy the ones offered by the manufacturer, or you can fabricate your own.
LocTite is also a good idea. Use the blue rather than the red, as the red is much more permanent.
As for sealant, you have multiple options depending on the application.
3M 5200 should only be used in applications where you have plans to
never remove a bedded item. 5200 contains a
powerful adhesive and once applied, is pretty close to becoming unremovavle.
3M 4200 has the same sealing properties as 5200, but without the adhesive, and is removable 'with difficulty'.
3M 101, LifeCaulk, and marine silicone are sealants that should be used in applications where you may want to replace an item someday. A good example would be sealing a wiring clamshell for a VHF with silicone.
Electrical components, antennas, and electronics need to be repaired, replaced, and upgraded occasionally, so you want to select a sealant that will allow you to do so when the time comes.
For help in selecting a proper sealant for your application, check out the 3M product line line here:
http://solutions.3m.com/wps/portal/3M/e ... e_Sealant/