The golden rule for different metals for water tubes is from less noble to more noble metal in flow direction. So from zinc plated steel to copper is much better then those steel tubes after copper.
Stainless steel seems to cause less or no problems with copper, i.e. the order does not matter.
But there is much less experience with stainless steel and copper tubes for domestic water distribution compared to zinc plated steel tubes.
An insulation/plastic part will prevent direct contact of 2 metals and can avoid contact corrosion. But it can not prevent pit corrosion, if noble metal comes before less noble metal.
If more noble tiny metal atoms/parts flow to and settle on less noble metal, the less noble metal can start a pit corrosion resulting in small holes. The pit corrosion is based on a local chemical element, i.e. the electric current does only flow locally around the settle-zone - any insulation between metal tubes upstream or downstream has little or no effect on this local corrosion.
This is why the golden rule is valid even if insulation devices do separate the different metals.
Steel boilers are made for upstream copper tubes since they are protected by a hard coating (f.e. ceramic) and by a sacrificing anode, which should to be maintained/tested regularly (decreasing size, limestone coatings, electric current f.e. 0.5mA).