I hope I am not intruding into your field of study with this question coming from a microbiology neophyte. I'm actually a photographer/photo journalist wanna be.
That being said, I'll go ahead with my question anyway, in all its non-scientific glory.
I have been making fermented products (for myself, family and occasionnaly friends) for a while now : miso, shoyu, sauerkraut, yogurt, pickles, etc. Recently, I've been wondering if i could not "boost" my yogurt with more beneficial cultures that you find in the probiotic pills and drinks available in pharmacy and grocery stores. I've been wondering first, if they would survive (i've done the research and i think 6/7 strains would), if they would grow happily during my 8h incubation at 42C, and, mostly, if they would over grow and maybe be harmful to my regular yogurt strains and mostly to myself. Is there a point where too much of those probiotic strains could be harmful to me ? how much can those trains actually multiply during a 8h fermentation at 40-42C ?
To be more specific, i would be using Probaclac, Adults. It has the following strains : Bifidobacterium bifidum, Bifidobacterium longum, Lactobacillus acidophilus, Lactobacillus bulgaricus, Lactobacillus casei, Lactobacillus rhamnosus, Streptococcus thermophilus.
I was thinking of mixing one pill with my 1 tablespoon of yogurt to innoculate my 1 quart of milk (and some added powdered milk protein).
Does anyone expect a problem ? should i not even try it ? should i try and see if it screws with my digestive tracks ? Can i get my yogurt tested somewhere to see whats in my yogurt after the incubation period ? Should I not post here anymore ?

Thank you very much for your time.
Simon