They use a mix of WW2 allied and axis( which make up the most ) style weapons while fight with the good army, but the thing that really amazed me is the creativity of the Metal Slug development team to mix up WW2, modern and future battle styles and the machine, weapons and characters. Although rebels are exact copy of Nazi Army but, it still just nice.
You gave the Di-Cokka a more WWII-like appearance; the tank itself is (if you don't know) based on the Sherman tank. It has the Rhino hedgerow-breaching 'tusks', as well as a .50 cal gun. Also in the turret are the 3 smoke canister launchers present on the Tiger tank.