Dear Sid The Kid:
You can just try to flush out the green coolant as much as you can, but don't worry about whether some leftover green coolant will mix with the Mercedes coolant (or Zerex G-05). Mercedes/G-05 coolant are designed to be "backward-compatible" with green coolant. In fact they do contain a small amount of silicates, which indicate that they were concocted with that backward-compatibility purpose in mind. The anti-corrosion compounds in green coolant (like silicates, borates etc...) are abrasives that may cause water pump seal to fail faster. Fortunately, the very low concentration of silicates in Mercedes/G-05 coolants will not affect water pump seals much.
However, if you use Dexcool coolant to replace green coolant, try to flush out green coolant as much as possible and add Dexcool. The green coolant will react with and deteriorate organic acid compounds in Dexcool, thus rendering it incapable of providing anti-corrosion properties anymore. You must flush out the mixture of Dexcool and leftover green coolant after one year. Try to do that twice (once per year). After that, you can just pour in Dexcool and keep it as long as 5 years in your car's cooling system.
Dear Mr. psfred: Dexcool does not have any of these anti-corrosion compounds: silicates, phosphates, borates, nitrites, and amines. The anti-corrosion inhibitor system in Dexcool consists exclusively of organic acid compounds. The biggest problem with Dexcool is that it may cause rust-like deposit in cooling systems if Dexcool level is low, or air is trapped inside cooling systems.
It's always better to stick with Mercedes/G-05 coolants.