Thanks for the speedy reply and good advice. I like the sound of Twin Lab's Super Probiotic but unfortunately already purchased some Threelac so I guess I'll give it a go first.
Just out of curiosity I ran my own little Threelac yoghurt experiment which produced some strange results:
I tried making a cup of yoghurt with skim milk and 1/2 a packet of Threelac (just to see if it'd work). I left it overnight with other yoghurt I'd been making and in the morning it was still in liquid form. I assumed like you say that the soil dwelling bacteria didn't take so well to milk and added a couple of spoonfuls of my yoghurt mix to the cup to see what would happen. Within about 5min the cup had foamed up and overflowed. Not too sure what to make of the mess but I think in the mean-time I'll leave Threelac out of my pro-biotic mix.
On a separate note I read that you're culturing your own kefir grains. Are you sharing/selling them as well? I'm trying to get my hands on some but it's been a little difficult since I'm living in Japan and most of the kefir here is sugary powder for drinks or in capsules.