As far as I know Tiff File format does not support background transparency but it does support layers. So if your Tiff file is a layer tiff file I would think you could turn off its background layer's visibility and save a png or gif file.
If the background layer visibility is set to OFF, there is no option to save a gif file.
Is the transparent background in GIF and non-interlaced 89a option available in CS5 version or any other image editing software.
There are two ways to save a gif. One is the document.saveAs() method using GIFSaveOptions as one of the arguments. The other is document.exportDocument() using ExportOptionsSaveForWeb as one of the arguments. Details about either are in the scripting guide.