Command Interface design for RTS

There are not so many combinations when you write them all down.

We just have a bunch of overloaded methods alike:

CmdArmy(cmdType)
CmdArmy(cmdType, Unit)
CmdArmy(cmdType, Coord)
CmdArmy(cmdType, Param)
CmdArmy(cmdType, Coord, Param)

Where cmdType is a key alike:

ctAttack, ctMove, ctStop, ctAbility, ctEtc ...

It's not OOP, but it's going along the lines of KISS quite well.

I'd use a simple base class. That allows you to write commands like this (You didn't specify a language so I'll write something C#-like, using some concepts from Unity):

abstract class Command
{
    abstract string Description {get;} //Maybe you want all commands to have a description in your UI
    abstract string Name {get;}//This is also for the UI. Neither of these affect your logic
    //...More things you want all commands to have

    abstract void Apply(UnitController unitController, Action onCompletedCallback);

}

class MoveCommand : Command
{
    override string Description 
    {
       return "This moves a Unit" //You'd probably have some logic here for localized strings instead of a literal
    }

    override string Name 
    {
        return "Move" //See above
    }

    private readonly Vector3 TargetLocation;

    //...More command specific members

    MoveCommand(Vector3 targetLocation)
    {
        TargetLocation = targetLocation;
    }

    override void Apply(UnitController unitController,Action onCompletedCallback)
    {

        //Something along the lines of:
        unitController.MoveTo(TargetLocation) //Unit controller owns the logic for movement 

        // --------------------Or---------------
        var pathFinder = unitController.GetComponent<Pathfinder>(); 
        if(pathFinder == null) 
             //Throw some exception
        pathFinder.Move(TargetLocation,onCompletedCallback); //Path finder owns the logic for movement

        //---------------------Or---------------
        var movementMangager = GlobalManagers.Resolve<IUnitMovementManager>();
        movementManager.MoveUnit(unitController,TargetLocation); //Some class owns the logic for movement.
    }
}