You're generally going to have to make decisions on QT based on the fish and your LFS. If you buy from a store that has all their fish individually separated, and have no signs of ich/stress personally I would keep that fish unstressed and drip him into your mature system.. If you have a fish like the Oto Catfish, these fish die a lot in transportation and need actual algae to feed.. This may be a canadict of no QT unless you supplement algae from your DT.
If you're unsure of the LFS put everything into QT and Drip it with Paraguard or a similar product. I also bleach certain plants to remove unwanted bacterials.
Don't let a $5 fish kill your whole tank, but don't kill a $50 fish die in QT because its stressed with no room to swim or establish itself. Ich can be treated pretty easily. Its a matter of keeping the fish fed and not stressed. Fish that are not stressed don't get sick.
Do you plan on keeping some filter media in your DT filter always seeded? I'd recommend having at least some seeded filter floss to be able to throw in for each new fish. SOME mechanical and all that existing biological will be good.
Most problems with a fish in cycle actually will occur closer to the end of the cycle. This is because Ammonia and Nitrite have both been toxic for longer period of time and both are still present because the significant water change (as required of all beginner tanks 80%) has not been done yet.
I just lost 22 neon tetras to Ich. I bought 12 from my LFS, 11 died, got 11 replacements, they all died. Now I only have one left. Luckily they were the first fish to go into the tank so there were no other casualties. I am going to wait a month before trying to add new fish.
You gotta be careful when you buy so many fish at once. Your filter does not have enough of a bacteria colony to support all those fish poo'ing. If you add new fish, feed less so the poo level stays the same.. If you don't you'll cycle the tank
Neons also need time to acclimate to temperatures. They like 74 and under naturally. So make sure your tank is near the tank temp as LFS because they are more adjusted to those ranges, for now..