In two recent playthroughs, I've picked up Dr. Fetus early on, followed shortly by other bomb-type pickups - Bobby Bomb, Mr. Mega. I seldom see these pickups, so I'm wondering if gaining Dr. Fetus increases the chances of also gaining the related items?
No, it does not. It merely has synergy with other bomb items. Your circumstances were a by-product of the Roguelike (Random Generation) factor, and not the Dr. Fetus jar.
Here is the Wiki page with all the items that stack with it.