I usually give the fish oil as the whole capsules in their food. They must like them because they will usually pick them out and eat them first.
I am giving a few different supplements to one dog, but that is mainly because he likely has IBD (supplements to help heal the intestines) but don't really give any to the other dog other than fish oil.
I do give probiotics, but from a pet food/supplement perspective (like every other supplement) it is definitely buyer beware, as it is un-regulated. Not all strains of probiotics are equally shelf stable. L. acidophilus, for instance, is notoriously unstable and won't last long at high temperatures or in moisture unless it has been stabilized in some sort of carrier (Pearls, for example), or refrigerated. So, buy ones that have a measured number of organisms (or CFUs - colony forming units) or some sort of "viability date". If it's just a weight of organisms, that really doesn't tell you if they are viable or not, they could be dead. And probiotics are not effective unless a sufficient amount is given on a consistent basis. You may have to adjust the amount you are giving if you are not seeing whatever results you are expecting. Probiotics is an area with a lot of potential but unfortunately it is mired in a bunch of marketing hype and the science is still developing on how to effectively use them. It's hard to tell what is real and what's not, unfortunately.