ADCs hit what they can reach safely. If the squishy Kog'maw is equally distant from them as the full tank Mundo, obviously it would be better for them to hit the Kog'maw. But 90% of the time this isn't going to be the case, because the enemy squishies will be sitting behind the front line, just as you should be.
Artillery mages like Xerath and Ziggs should AIM for the squishies, but otherwise focus on poking the front line and deterring an engage with their cc. These types of champions like to build Liandry's and Void Staff a lot, so hitting the tanks isn't a big deal for them.
Burst mages like Syndra and Veigar should play more like assassins, in that they should save most of their abilities for squishies who overextend or get caught out of position. However these types of mages still have valuable cc that they can use on the enemy front line should they get too close.
Everyone else can hit whoever they want, because it doesn't matter.