for yeast problems, there is scientific evidence that some strains of Lactobacillus plantarum are effective against Candida albicans - there's one isolate that completely obliterates Candida but other strains get Candida down to levels that are controllable by the remaining gut microbes.
when dh started his cleansing regimen a few months ago (gah - he didn't ask what everything was for...), he started on the Nutrition Now PB8 product containing L. plantarum along with 7 other microbes. when i stopped my UltraFlora Plus product, i had some intestinal issues until i spiked my kefir with a capsule of the PB8. now everything is just hunky dory. i imagine one could do the same with yogurt to see if it were helpful. and PB8 is on sale this month through Frontier Coops. PM me if anyone wants me to send a capsule or two through the mail (since we have some on hand anyway).
i also read a study that showed that various essential oils are selective against unfriendly microbes but not that harmful to desirable critters (only 20% loss of good ones but 80 to 95% loss to the bad ones). moreover, the oils survived digestion. might be worthwhile spiking beverages with a few drops of thyme oil, cinnamon oil, or oregano oil. of those 3, thyme oil and cinnamon oil require only very small concentrations - and the thyme and oregano oils have been shown to be effective on candida, not sure about cinnamon oil (which is good against pathogenic E coli). i haven't been able to sit down and figure out the possible dosages though...