How to use MDI technology if you don't want to use TMDIForm as your mainform of the application?
You want to use TMDIForm that you open/create somewhere during runtime and is not the mainform of your application, but Delphi doesn't allow creating TMDIChild if the TMDIForm is not the mainform of your application.

When I wanted to use TMDIForm that wasn't the mainform of the project and then create TMDIChild in that form, I got an error. Delphi didn't allow creation of TMDIChild as long as the mainform wasn't the TMDIForm. So I tweaked the FORMS.PAS a little bit.

1st. You have to add a global variable in Forms.PAS.

----- snip -----
interface

....

var uMDIForm : TForm;

implementation

...
----- snip -----

2nd. You have to change CreateWindowHandle and DestroyWindowHandle procedures a little bit.
Here are the code posts of the changed procedures.

----- snip -----
procedure TCustomForm.CreateWindowHandle(const Params: TCreateParams);
var
  CreateStruct: TMDICreateStruct;
begin
  if (FormStyle = fsMDIChild) and not (csDesigning in ComponentState) then
  begin
(* add the check if the uMDIForm is set and if it is TMDIForm
   if it is not, then raise an exception *)
    if ((uMDIForm = nil) or (uMDIForm.ClientHandle = 0)) and
       ((Application.MainForm = nil) or (Application.MainForm.ClientHandle = 0)) then
      raise EInvalidOperation.Create(SNoMDIForm);
    with CreateStruct do
    begin
      szClass := Params.WinClassName;
      szTitle := Params.Caption;
      hOwner := HInstance;
      X := Params.X;
      Y := Params.Y;
      cX := Params.Width;
      cY := Params.Height;
      style := Params.Style;
      lParam := Longint(Params.Param);
    end;
(* check if uMDIForm is set if not then use the MAINFORM of the application
   as TMDIForm *)
    if uMDIForm = nil then
      WindowHandle := SendMessage(Application.MainForm.ClientHandle,
                                  WM_MDICREATE, 0, Longint(@CreateStruct))
(* else use the form specified in uMDIForm *)
    else
      WindowHandle := SendMessage(uMDIForm.ClientHandle,
                                  WM_MDICREATE, 0, Longint(@CreateStruct));
    Include(FFormState, fsCreatedMDIChild);
  end else
  begin
    inherited CreateWindowHandle(Params);
    Exclude(FFormState, fsCreatedMDIChild);
  end;
end;

procedure TCustomForm.DestroyWindowHandle;
begin
  if fsCreatedMDIChild in FFormState then
  begin
(* again check if uMDIForm is not set and if then destroy TMDIChild from
   mainform *)
    if uMDIForm=nil then
      SendMessage(Application.MainForm.ClientHandle, WM_MDIDESTROY, Handle, 0)
    else
(* else destroy the TMDIChild in uMDIForm *)
      SendMessage(uMDIForm.ClientHandle, WM_MDIDESTROY, Handle, 0);
  end else
    inherited DestroyWindowHandle;
  FClientHandle := 0;
end;
----- snip -----

Now before you create any TMDIChild forms you have to set uMDIForm variable to that form that you have set as TMDIForm.

Hope this is usefull to anyone.

If someone has any idea how this could be done by passing a new variable to TApplication or something similar, please contact me.