You do need to get all of the fish into the tank pretty close together. there are two reasons for this. First, that is why we do a fishless cycle at about 3 ppm of ammonia. it allows us to stock the tank fully on day one. however, day one is actually a couple of days not one. However, there is a more important consideration. This has to do with quarantining fish. if you get all your fish from the same source all at once, your tank is a quarantine tank. However, if you use two sources, then you are mixing fish from them and if one source has an issue while the other does not, you get to share the issue between all the fish.
If you want to order online for next Tuesday delivery, dose the same amount of ammonia every 2-3 days until the fish come. You still must do a big water change right before the fish go in.
The thing is if you get the fish from all one source, the odds are any issues there might be will usually be the same for all the fish from that source. Most stores use central systems or mix equipment between tanks. However, two sources may have different issues and then you are dealing with two problems not one.
Q tanks are important if one adds new fish over time. One new sick fish can wipe out a tank you have worked to get established and stocked. You would not like my normal practice. Tank raised fish are Q'd for 30 days and wild one's for 90 days. This means they must go for that long without any issues before they go into a permanent tank. If I have to treat the fish, when/if they recover, the Q period starts anew from there. When I sold at events and would purchase some fish specifically to sell, I would bring them in so I had at least 3 months in Q before I would sell them.
As you are in the hobby for longer, you will become attached to some fish you have had for a while. Nothing sucks more than harming your favorites because you did or could not quarantine something new before you added it.