Jab out of shield comes out just as fast as Olimar's grab at pointblank range. At about ftilt range away though, a jab will hit the opponent more quickly. They can just hold shield to avoid the second hit of the jab though, so what's the point. *shrugs*
Another good option out of shield is to jump cancel an upsmash. If they land directly next to you or are possibly using a rising aerial, you can jump out of shield->upsmash to hit them where a grab wouldn't work, and perhaps where an aerial wouldn't hit either if they're rising too quickly for you to be able to jump and do an aerial in time.
To answer your second question... yes, aerials out of shield are great (if you're at least somewhat sure they'll hit, that is)- especially nair.
One last thing. Use aerials out of shield sparingly. You don't want to get caught in the air.