But... I'm running into a little problem. I have created a Panel called panelLogin which I render to the screen with wsApp.app.ui.setCard. After rendering I want to add a FormPanel called formpanelLogin:
Fit layout panels need to have dimension in order to 'fit' an item inside them, so you would need to give panelLogin a height in order for anything to be displayed in it (fit panels should have only 1 item, although they can have multiple dockedItems).
Having said that, I do think you found a bug. In the below example, the auto layout panel has it's button replaced correctly, while the fit layout panel has it's button removed, but the new one is not rendered.