Is it possible to change the file format of Grab.app?
Doesn't seem to be possible. Even the limited Help for Grab.app suggests using Preview to convert away from TIFF. I presume you need "Timed Screen Captures" otherwise you would use the keyboard shortcuts?

Note that you can take the grab, then copy the image without saving (CMD+A, CMD+C), then ALT+TAB to Preview.app, then CMD+N, then save. Not very elegant.

You could also just deal with Grab.app saving as TIFF, but then set up an Automator action that watches a folder and converts TIFF to whatever you want each time a TIFF shows up in the folder. This ought to be fairly easy and transparent way to use Grab.app and get the file formats of your choice.