I hope he's OK. I would do frequent water changes with temperature matched, treated water, probably two 25% changes today, and then 50-60% every day hereafter.
I wouldn't add any more regular salt, whether table or aquarium. If this is dropsy then Epsom salts can help with the fluid build up to make him more comfortable. I would dissolve about 1/8 teaspoon per 5 gallons and add that to his tank, after you've done water changes today.
Epsom salts will help with the fluid, but won't cure him if this is dropsy. Regular salt doesn't really help with that at all, it may have helped him breathe a little better if your nitrite levels where up, but I advise against using any more regular salt at this time.
Don't beat yourself up, please. We've all gone through times like these, every one of us. You did great getting him a heater, I'm sure that helped him feel better.
I would keep his water temperature at around 76 degrees, just warm enough to help him feel better, but not so warm it helps any bacterial infection he may have.
When you can, go get the API Master kit to test your parameters. Don't get the test strips. They are very unreliable and most often give inaccurate readings. Besides, the strips are way more expensive. The API is a little more up front but lasts for months, even testing every day, whereas the strips won't last a month.
Good job, not washing the decor, you no doubt brought some of the beneficial bacteria (bb) over to the new tank. When you clean the filter, when the biofilter media looks a little gunky, always use some tank water to gently swish the biofilter media; sponge, floss, poy, or bio balls, whatever, because you want that bacteria to stay in there.
I rinse out the sponge or foam that is there to filter out particles with tap water, but I always soak them a bit after that in some water with dechlorinator in it before putting them back in the filter with the biofilter.
Prime is an excellent water conditioner. It's cheaper than most, too. Prime not only removes chlorine and chloramines, but it also detoxifies ammonia and nitrite.
This is very helpful should you detect a spike in either or both of those toxic byproducts.
You can even dose 5 times the normal dose of Prime when you find a spike, and this gives you some breathing room to do the necessary water change at a more convenient time and not in panic mode. You still have to do water changes to remove them, however, within 24 hours of the emergency dose as that's how long the detox lasts.
Keep your eye on the other two guppies and watch for signs of problems. Keeping their tank water pristine goes a long way in helping them recover from stress, and even fight off infections Watch for behavior changes, often the first sign of a problem.
If you can post photos of your sick guppy that would be helpful. When you get your test kit, be sure to test for ammonia and nitrite every day and do water changes as needed to keep your parameters at: ammonia 0 ppm, nitrite 0 ppm and nitrate 20 or less. Test the ph, too. Post those results for us when you can. We're all pulling for you.
Please update us when you can.