You can use the following:

For copper etchant: use APS 100 (see Kirt's paper). It does not attack
nickel.
For Cr etchant: mix 1g of NaOH and 2g of K3Fe(CN)6 in 8ml of H2O. I think
this does not attack nickel.

I hope this info would help.

Daniel Sang-Won Park