You should be fine to have Nickel and Silver in the same loop.
On the Galvanic Index, they are pretty close together, not much potential for corrosion.
http://martinsliquidlab.files.wordpress.com/2012/01/corrosionplating.png?w=614
Source
http://martinsliquidlab.org/2012/01/24/corrosion-explored/
The reason why Silver is recommended against when Nickel is present is because of the common water-cooling metals, they are the furthest apart on the index. That and if something ever goes wrong (EK's Nickel Plating incident) they usually find a scapegoat, EK's was that the use of distilled water and not their branded coolants is what caused the problem.