Once the car was warmed up, did it run fine?- that would tell you if it was stumbling because it was cold.
The way you have this phrased it sounds like car didnt run at all. You put in a little Seafoam. Car now starts, but stumbles when given gas. Is that accurate? We might need some more details on whats going on with your car.
Seafoam works great with what its intended to do. It really will clean out all the gunk in your system, but sometimes its more than you want with older engines. Sometimes that gunk seals up your rings and once you clean them out, you start having blow by or similar. Putting a "tiny bit" probably did nothing for your engine and the hard idling issue would have been present without the Seafoam additive.