You could put it in their food if you really want to without it causing problems. Sometimes it can help, but most of the time it doesn't seem to do much at all unless the chin has a digestive issue from stress or something else. But, it won't hurt and it could definitely help if the chins are stressed or end up overeating or have another problem.
You can use pretty much any brand of acidophilus or probiotics. I'd stay away from anything that is chewable or has sugar. I prefer the PB8 brand because it is shelf stable, has a pretty decent organism count and is fairly readily available around here. There's a lot of different brands out there. If you have a vitamin store local to you, go there and look around at what they have. They may have probiotics in a refrigerated section, so you may want to ask for help so you can look over everything they have available. And, there's always Amazon.com if you can't find something locally.