[TZ@PCI]

First I would check to see if the pumps are the new (NSS)non-stainless steel versions. If they are then just regular maintanence is required.
The Stainless steel does not rust, it is only inert metal ions being attracted to the stainless steel do to the electromagnetic field being generatedby the pump.

The thermal plate will not be affected if run externally, it won't come into contact with water.
Regular silicon will work fine, you do not need to use an epoxy.
The stainless steel version will work fine if you keep the impeller clean. Simply clean it with steel wool to remove any metals that have been attracted to the surface.
If you are using deionized water the level of metals are lower thus reducing the build up of metal ions.

If your new pumps are our old style let me know I will gladly ship you out our (NSS) Impellers.
Sincerly,