I think the key here is patience. You really do need to take your time, months in my view, to set things up.
Also, when I choose my fish I always spend at least ten or more minutes watching the behaviour of the fish in the tank at the LFS
and take a very good look at the quality of the fish. If I see any signs of disease or fin rot, etc I do not bother with it. That's the only way you can do it in my view.
If you have a spare tank, you could move the inverts from the main tank into it, and then treat the main tank with some more harsher remedies for your diseases. Whitespot/ich is a prick of a thing, and there are some good remedies which work on fish but nuke your inverts and corals.
If it were me, I would not add any new stock to your tank now and concentrate of resolving each problem one by one. Make sure you have sufficient good bateria levels in there, do water changes using treated water (if possible) with a monthly wait period between them and if you want to use remedies, make sure you come back here and ask first.
The bottle labels are not always right, and they don't take into account your specific situation, but the collective input and experience here can at least guide you better than what a 2c label can.