Poking at the internet a little, it appears that if you use a standard 70mm Hydra launcher, the rockets stick out the front about the length of the warhead section, give or take. They do seem to have modified launchers that fully enclose the rocket's length, similar to the unguided versions. As far as the loadout, I'd presume if you can find reference to a old style unguided rocket the guided versions would be a 1-to-1 replacement.
As far as why you would want this, my guess is price. The 70mm Hydra rockets are pretty much dirt cheap, at least compared to the Hellfire, and if you would guarantee the hit, one Hydra would take out any soft skin vehicle, and if you are carrying a 19 round pod, that means you have 19 kills, where the unguided rockets would be a handfull of targets at most. Just my 2 cents here.
John