For skimmer placement, the water level in the compartment has to be absolutely constant in spite of water evaporation. So typically, it's the 2nd compartment.
Many older 1st compartments are designed for filter wool, with wet-dry media below. That can take up quite a bit of space. If you plan to convert to using filter sock, removing the baffle between the 1st two compartment can free up quite a bit of space if you need it to fit in a bigger skimmer.
You can fit a few stalks of mangroves onto a floating piece of Styrofoam in the last compartment.