how to add a fill layer in photoshop programatically

This may be of use:

You can define a new colour with new SolidColor()

   // define fillColor 
   var fillColor = new SolidColor();
   var myColour = [57, 181,74];
   fillColor.rgb.red   = myColour[0];
   fillColor.rgb.green = myColour[1];
   fillColor.rgb.blue  = myColour[2];

and then fill your path by adding myPathItem.fillPath(fillColor,ColorBlendMode.NORMAL,100,false,0,true,true);

// Switch off any dialog boxes
displayDialogs = DialogModes.NO; // OFF

var originalUnits = app.preferences.rulerUnits;

app.preferences.rulerUnits = Units.PIXELS;

// call the source document
var srcDoc = app.activeDocument;

create_path("bar-path");

function create_path(linename)
{

   var points = [    
   [100, 100],
   [200, 100],
   [200, 110],
   [100, 110]
   ];


   // create the array of PathPointInfo objects
   var lineArray = new Array();

   for (var i = 0; i < points.length; i++)
   {
      lineArray[i] = new PathPointInfo;
      lineArray[i].kind = PointKind.CORNERPOINT;
      lineArray[i].anchor = points[i];
      lineArray[i].leftDirection = lineArray[i].anchor;
      lineArray[i].rightDirection = lineArray[i].anchor;
   }


   // create a SubPathInfo object, which holds the line array in its entireSubPath property.
   var lineSubPathArray = new Array();
   lineSubPathArray.push(new SubPathInfo());
   lineSubPathArray[0].operation = ShapeOperation.SHAPEXOR;
   lineSubPathArray[0].closed = true;
   lineSubPathArray[0].entireSubPath = lineArray;

   //create the path item, passing subpath to add method
   var myPathItem = srcDoc.pathItems.add(linename, lineSubPathArray);

   // define fillColor 
   var fillColor = new SolidColor();
   var myColour = [57, 181,74];
   fillColor.rgb.red   = myColour[0];
   fillColor.rgb.green = myColour[1];
   fillColor.rgb.blue  = myColour[2];

   //fill the path so we can see something also
   myPathItem.fillPath(fillColor,ColorBlendMode.NORMAL,100,false,0,true,true);

   // deselect path
   deselect_path();
}

// switch back to normal
app.preferences.rulerUnits = originalUnits;


// Set Display Dialogs back to normal
displayDialogs = DialogModes.ALL; // NORMAL


function deselect_path()
{
   // =======================================================
   var idslct = charIDToTypeID( "slct" );
   var desc76 = new ActionDescriptor();
   var idnull = charIDToTypeID( "null" );
   var ref63 = new ActionReference();
   var idPath = charIDToTypeID( "Path" );
   var idOrdn = charIDToTypeID( "Ordn" );
   var idTrgt = charIDToTypeID( "Trgt" );
   ref63.putEnumerated( idPath, idOrdn, idTrgt );
   desc76.putReference( idnull, ref63 );
   var idselectionModifier = stringIDToTypeID( "selectionModifier" );
   var idselectionModifierType = stringIDToTypeID( "selectionModifierType" );
   var idremoveFromSelection = stringIDToTypeID( "removeFromSelection" );
   desc76.putEnumerated( idselectionModifier, idselectionModifierType, idremoveFromSelection );
   executeAction( idslct, desc76, DialogModes.NO );

}

As for rectangle being too large: What resolution and units do you have the psd file set to? The script below will switch to pixels, and set the resolution of your document to 72dpi

As for doing a stroke to replace the line. Well... you've got options. You can do a brush stroke with a two point line:

var idStrk = charIDToTypeID( "Strk" );
var desc105 = new ActionDescriptor();
var idnull = charIDToTypeID( "null" );
var ref35 = new ActionReference();
var idPath = charIDToTypeID( "Path" );
var idOrdn = charIDToTypeID( "Ordn" );
var idTrgt = charIDToTypeID( "Trgt" );
ref35.putEnumerated( idPath, idOrdn, idTrgt );
desc105.putReference( idnull, ref35 );
var idUsng = charIDToTypeID( "Usng" );
var idPbTl = charIDToTypeID( "PbTl" );
desc105.putClass( idUsng, idPbTl );
executeAction( idStrk, desc105, DialogModes.NO );

However, I don't think that's what your after. You can turn your path into a shape and then have a stroke and fill colour assigned. However... you need a minimum of 3 points for that to work.

As a suggestion, instead of a path create a shape from start - which oddly will work with a minimum of two points. But I have no idea how to do that in code!

There are two lessons I have learned form translating the solution by Ghoul Fool into Python:

1. using COM in python, one can make a lot of mistakes that go unnoticed, otherwise than cryptic error messages far beside the point. Thies include syntactic errors such as missing/redundant parentheses and wrong capitalisation.
2. solutions that work with COM in python do not work the same with the Python-Photoshop-API and vice versa, even though, following the source code of the library, apparently exactly the same thing is happening. I have not always been able to divine solutions that work both ways.

Here is, for those who have come to this question later, what I have come to to get a shape layer with a bar that can be changed in height afterwards. The action that is called is nothing more than a recording of New Fill Layer from the layer-menu (CS6):

def makeBar(doc):
    app = doc.parent    
    app.preferences.rulerUnits = Units.Pixels
    
    def point(x, y):
        result               = Dispatch("Photoshop.PathPointInfo")
        result.Kind          = PointKind.CornerPoint
        result.LeftDirection = result.rightDirection = result.Anchor = (x, y)
        return result

    points = [    
              point(100, 100),
              point(200, 100),
              point(200, 110),
              point(100, 110)
              ]

    lineSubPathArray               = Dispatch("Photoshop.SubPathInfo")
    lineSubPathArray.Operation     = ShapeOperation.ShapeAdd
    lineSubPathArray.Closed        = True
    lineSubPathArray.EntireSubPath = points
    
    doc.PathItems.Add("bar-path", [lineSubPathArray])



app = Dispatch("Photoshop.Application")
doc = app.Open(self.fileName)

As for my second question: The unit of the path's cornerpoints are invariantly in Points, no matter what he rulers are set to.