Guys, I hate to be rude, but I'm not worried about the sick getting sick. I've had fish for half my life - 12 years almost. I had over 30 tanks at one point when I was rescuing fish. I brought back an Oscar that had severe HITH to pristine health and he just recently died at the age of 10. Okay, not really recently, it's been a year.
I know how to treat illnesses and have been successful with a wide variety including ich in a scaleless fish tank. The only illness I failed with was a betta I got from Craigslist that had these strange growths and at the forum I was a member of at the time, we decided they were tumors of some sort. The fish still lived another year, but always with the growths.
The three fish I have now are active, taking their new food without reservation, and seem excited about being in a calmer place than the pet store. I'm still getting wiggle dances from the initial betta and the other two have been more active than they were at the store. I got them for their colors and they are brightening up.
If disease does come about, it would be a lot easier to treat the entire tank than to try dosing for those little cups. That would be nearly impossible. I looked for healthy fish when I chose the bettas, as I always have. If they do get sick, then I'll deal with it and it will only take a few weeks out of my life.
As for getting more fish, I do agree with you. However, at this time, I know I can't add as many as I truly want because the tank is fresh and hasn't had a chance to cycle. I helped it along a little by moving some filter material from my turtle tank, which has an Oscar and pleco, but it will still take time and I don't want to put too many fish in. Any new fish will be quarantined as I will assume my original three to be healthy, whether they are or not. Another reason I have not gotten more fish is because it was a spontaneous buy and I was at the tail end of my pocket money. I will be working this weekend and making a few hundred dollars, so next week I will have plenty of money to buy another set of fish and a 10 gallon as a quarantine tank. My new filter should also be coming in on Monday. Right now, the new betta tank is running without a "real" one. I have the filter material both in a mesh bag in the corner and some in a little air pump powered filter you can use for tanks less than 5 gallons. I also have an air stone going to help with the gas exchange and the four plants, so it shouldn't be too bad for the weekend unless the plants decide they want to die.
I'm also running low on my test kit fluids, so I ordered more. It should be in with the filter and for now I am doing 10% water changes in the new tank just in case. I can't imagine the ammonia being very high at this point as the fish are small and the plants are probably helping. As I said, I am very experienced and joined only because I like forums and talking to people who have common interests.