Is it possible to change the file format of Grab.app? I know that you can change the format used by ⌘⇧3 and ⌘⇧4 (SE answer and external source), but is it also possible to change the format of Grab.app (uses TIFF)?
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.