Lol well we're not all
experts per se, but it helps that we have experience in the field
as far as fin curling, they can straighten out depending on which type make them curl. If they are curled because of hard water, they will always stay curled, no matter what. But if they were curled from the ammonia build up in cups, or in tanks then they can uncurl a bit or all the way depending on how bad, when they are given new fresh water frequently. So it's not the store water, but it's the ammonia build up in the cups and I have one boy who had that very same thing happen to his ventral fins but it hasn't uncurled all the way, I've had him for about 5 months now. It's certainly straightened a bit but it will never be fully straight again.
Yeah if you test and see your ammonia get above .25ppm then go right ahead and do a small change, 25-50% should be just fine if you've got a siphon to use.
And as far as plants, it depends on how many you have. It will help some if you do have a few in there but it depends on the size of the tank, the fish and how many plants. It's not just fish waste that goes into ammonia, it's also dissolve organic compounds which can come from plants/water/fish. So the more plants you have the better, yes. And actually with plants they do act as a buffer against harmful levels of the three; ammonia, nitrite and nitrate, if you have enough.
If you have a filter they will actually help with the Nitrogen cycle and getting it going, usually if you get plants from another tank they will already have Beneficial Bacteria on it BB (that's what your growing during the cycle), and so it helps give the cycle a "kick-start" and then of course protects the fish during the actual cycling process where the BB are being grown
Does that answer your question?