Before you start stuffing the little dears with everything known to man, which can do more harm than good, take a breath. Okay, with me? Now, is it every poop that is brown. loose and stinky? Every poop, every time? Most likely you'll realize that it's not. Most likely you'll realize that it's just sporadic one from each chick every day. The advice you got about not feeding medicated feed if you've had them vaccinated is pretty much spot on although it does depend on what they were vaccinated for. In most cases, medicated food is not indicated or recommended for vaccinated chicks. Vaccinated or not, I never use medicated food anyway. Instead I toss a chunk of sod, dirt, grass, roots and all, into the brooder. It helps build up their immunities slowly, gives them a little natural grit, teaches them that scratching is FUN and productive, and they love to dust bathe in the dirt they knock off.
I'd be willing to bet that what you are seeing is "cecal poop", very normal. I know it looks scary, but all chickens do it and chicks are no exception. It simply means their digestive system is functioning. I can be out in the coop and know when a chicken in the run has just dropped one.
So relax, observe, and then make the call about what you are seeing. If you are still really concerned, then a re-post in the diseases and emergencies forum would be in order.
Welcome to BYC! And as I said, observe but relax....stressing out will keep you from fully enjoying your chicks.