I think it honestly depends. I used to have "Big red", whom I got when he was older (it was before you found the youngsters in stores). He lived in a 1g his whole life, and was LUCKY to get a once a month full water change. He thrived- I had him for 2 years at least, I think it may have been 3- and he was probably a good six months old when I got him.
That said, some of the new finnage (Halfmoons esp.) are thought to be MUCH more susceptible to fin rot and the like. I'm fighting fin rot with my boy Ghost, but I blame myself for that one.
I think, honestly, that the filtration (plus finnage) are really crucial- go ahead and yell at me, but Ghost and Delph don't have filters (yet). Even Holly's filter is just one of those cruddy discard-a-filters. I plan to get the boys better homes so I can put a filter on them. Awesome- he's never had a sick day in the time I've had him, but he's had a filter of one sort or another for 80% of his time with me. I do think we over clean sometimes, but depending on set up, you do need to clean a lot.
I admit, I don't follow the "rules" this forum loves

But it does NOT make me some horrible person, either. And I love my fish as much as anyone else- and I know I'd be so upset if Ghost, Delph, Awesome, or Holly died- it was bad enough when Ghost got fin rot and the 10g got parasite attacks all at the same time!
I can't say I know exactly what you're going through, Allie, but I know I'd be in so much pain to have lost as many as you have. I just lost two of my three platy girls- add that with Ghost and the rest of what's going on, and I'm really not sure if I'll be keeping anything but the bettas once this group of fish pass.
I'm sorry this happened to you- I really do wish I could offer you an easy fix, and some way to make everything better now.