Ryan.
what specific ERS switches are you referring to in this case?
But to Answer your question. When introducing an ERS switch to an existing stack the best way is to ensure the switch is running the same FW/SW.
As mentioned above, ERS4500/4800 and ERS 5500/5600 supported mixed stacks within their family. However, those products are EoSale. Anyone looking to build out their current install base should be looking at the ERS4900/5900 and migrate away from 4500/4800/5500/5600. The new 4900 and 5900 do not support mix-stacks, but there are ways to introduce new switches with a migration strategy and transition period.