I wouldn't bother using gelatin unless you have a specific reason. Keg and if it doesn't seem to clear very well or has a yeasty flavor, you can add gelatin in keg, wait a few days, and pour off a pint or so. Keep in mind that there will be gelatin residue on the walls of the keg, and if you plan to take it to events you will need to let the gelatin that sloughs off the walls settle and be poured off before serving. If it will sit in your kegerator, no problem.
I haven't noticed head retention issues with beers I have added gelatin to. If you are concerned, add a smaller amount of gelatin than recommended and see how it works for you.
For stouts, I would think it OK to let the keg rest cold for several weeks or so before drinking it since the style is generally more ageable anyhow. After a month or so it should be relatively 'clear', though the darkness may make that imperceptible. My ultimate recommendation is no gelatin until you absolutely feel you have to try something.