I really can't tell that either of those products works very well at all. Certainly not good enough quick enough to save guppies infected with columnaris. It might be possible they can help slow or stop the spread of the disease.
In any case the Primafix is for fungi and columnaris is a bacteria, though it confuses people because it looks like a fungi. So one wouldn't expect the Primafix to work.
The Melafix is supposed to work by oxidizing the bacteria. If I was going to go that route, I'd do a hydrogen peroxide bath instead. At least that works sometimes.
Columnaris is a gram negative bacteria. So basically you have to use something that works on them. That list gets fairly narrow in aquriums.
Copper sulfate works well in the water but your invertebrates will die faster than the bacteria.
That leaves us with terramycin, which works well, especially if they are still eating and you can get a medicated feed into them. If nothing else, it protects the ones that aren't already sick.
Terramycin can also be used in the water, and I believe it won't kill your other species but I don't know.
Other antibiotics that should work would be Furan or Acriflavine.
So get a product with those and maybe you can save your fish.
Now antibiotics are just part of the treatment. First get rid of any charcoal or it will absorb the antibiotic. Then, partially change the water, and keep doing partial changes daily until the epidemic breaks.
If your particular other tank dwellers will stand it, salt helps the guppies breath. Columnaris kills by blocking the gills and the fish suffocates. Adding salt is like putting a person with pneumonia on oxygen. It helps them breath and buys you time for the antibiotics to work. I'd put at least a teaspoon a gallon in and if the snails die, well I like my fish more than my snails. Or remove a few snails to some container until this is over. They're just snails it's not like they require a huge filter running every moment to stay alive. Probably move the others too. But that's me. I love my guppies and like the others.
If the others are your priority, get some clove oil out, capture the guppies and anesthetize them and put them in the freezer and get it over with. Sorry if that sounds harsh but if you don't use something that could work, you've already made the decision to kill most of them by suffocation after a period of illness. Being knocked out and then frozen is a much nicer way to go.
So water changes with vacuuming, add salt. And try and find one of the antibiotics that would at least have a chance of working. If the label doesn't list one of those, I wouldn't even bother trying it unless it specifically mentions columnaris or gram negative bacteria.
And it is one of the most spreadable diseases so it is important to disinfect things like nets and such between fish.
For some reason, it's actually fairly difficult to find antiotics that will work in many LFS.
Aquarium Pharmaceuticals has some good products, and have both the Terramycin and Furan available. See:
http://aquariumpharm.com/Products/Product.aspx?ProductID=65
I've had the most experience with the Terramycin, it seems to work well if the fish isn't too far gone, it just takes some time for any antibiotic to work and sometimes you just can't keep the fish alive long enough until the antibiotic works. That's where the salt comes in. Anyway the Terramycin doesn't seem to mess up your filtration so you don't cure the disease only to lose them to an ammonia spike. That's what I'd try.
Marv