You're very welcome, glad to be of help. :)
10 gallons is certainly not too large for a betta at all. Thats a common misconception actually, that bettas prefer smaller tanks and larger ones bother them. No tank is ever too large when decorated properly...and because that can be a little on the expensive side, not everyone does, and then when they put their fish into their larger home they become stressed with the lack of cover...leading to the misconception.
I'd like to point out though that not all bettas are like that. Some do just fine with a couple of plants and a cave or two in a larger tank. But it does happen quite frequently, and I just wanted to explain that its usually because of a lack of proper cover(some do like more then others as well...a shyer betta especially)rather then the size of the tank. ;)
Just plant them both really well. Live plants would be fantastic(its okay if it looks a tad bare of course at first, the plants will grow in...just offer something floating like hornwart or Java moss. ;)), maybe a little driftwood to give it that natural look.
There is a fantastic amount of info on live plants here on this section of the forum:
And here is a wonderful thread on how to do a full Naturally Planted Tank, if thats a project you think you may want to take on with the live plants.
Ah, never be sorry for questions! Questions are how you learn. ;)
Your fish didn't get ich because of a lack of salt. They likely got it because of their compromised immune systems in the cold water.
For bettas you should only use AQ salt as a treatment for some kind of illness(Ich/parasites, bad finrot...). I believe it may be alright and possibly beneficial to guppys, which is why Im sure your husband suggested it, but for bettas its actually harmful and dangerous if used regularly over time.
A much safer and better alternative as a regular additive for bettas is IAL(Indian Almond Leaf). It has a LOT of beneficial properties, including antifungal, and basically does what the salt is said to do....but better, IMO.
When you're doing some kind of treatment, 100% per day is not too much for a short period of time.....but regularly its not a good idea. You don't want TOO sterile of an environment, there is such a thing as too clean, but there is such a thing as too dirty as well and its really hard to hit that perfect middle ground in smaller tanks.
As a regular maintenance in a 1 gallon, I would say that 2 50-70%s and 1 100% per week should be about good.
The 100% change today probably won't hurt anything. Its possible some of the medication got in the water anyway.
Just be sure when you do a 100% change though to re-acclimate them back into the fresh water. Float their cup or container for about 15 minutes, pour out a little of their water in the cup/container, then add a little of the new. Repeat the process a couple/few times every ten minutes or so and then release them. :)