Underplating question

I’m trying to get a handle on the various methods of what to plate over what and why.

Gold over silver directly seems to be acceptable for some, and absolutely forbidden by others. Gold over copper, the same. Silver over copper, the same.

There’s often talk of using nickel as a barrier plate underneath a variety of different platings.

My question is this: are there alternatives? With at least 10% of the population being allergic to nickel, it seems like a risky proposition to use it as an underplate on anything that might have skin contact. So are there options for a barrier solution that’s not problematic?

Copper will work its way to the surface of gold… so if you gold plate directly over sterling the copper will come to the surface and oxidize just like sterling does… that is why people either nickel plate or rhodium plate or Palladium plate first these are barriers which the copper can not get through… thus keeping the gold from oxidizing.

So palladium or rhodium will work as a barrier over copper the same as nickel?

What about silver? I’ve heard you don’t rhodium plate directly over silver. Could you copper over silver and then rhodium over that (or gold over that)?

I’m just trying to figure out the best method for eliminating the nickel from the equation, since nickel is, by and large, exceptionally problematic in worn jewellery. And when we’re dealing with plating layers that can wear off, I wouldn’t want that in the equation.