I have only used pro-biotics in cheese making. I purchase dedicated vegan yoghurt bugs to make my yoghurt. They keep frozen and use from frozen. 1 packet that costs around GBP12 lasts us about 2 years. It says for 100L of yhourt, but we easily get twice that. we make a 2L pot every 10-14 days.
I have to be careful with the probiotics. Most are grown in dairy and labelled vegetarian. Some less honest say vegan, but if you read the small print, it says grown in dairy. these are grown in something that is not daity and not soy, and those are the ones I have to hunt out. I use this company (not that I am suggesting them, it just gives you the bugs I use)
Vegan Non-Dairy Yoghurt Culture (Makes 100 Litres)
It consists of specifically selected strains of:
Streptococcus thermophilus
Lactobacillus delbrueckii ssp. bulgaricus
and the probiotics
Bifidobacterium animalis ssp. lactis
Lactobacillus acidophilus
The 'sugar' are natural sugars. there is nothing sweet about our yoghurt. It is as plain as it can get and there is not sweetness to taste - the bugs need a certain level of sugars to grow in. We have found that they will grow in half what people say they need.
you could always just use live yoghurt - just premix with some of the soy milk first to a thinish mix and add.
By the way, we don't bother boiling the soy milk and allowing it to cool unless it is homemade. An unopened long-life container should not have any need to be pre-boiled, in my view! We've done it that way now for 8 years without issue. We also keep the yoghurt in one big 2L pot with a sealed lid and only have 1 rule. - clean spoon every time. It will last about 2 week without issue. In fact we have been known to make a pot of yoghurt and not open it - put it in the fridge and go on holiday on the grounds that if it is mouldy when we get home - we have only lost 2L of milk. We have never had an issue.