I personally would go 10 gallons minimum. You can QT everything but you cant medicate/ treat everything the same. Inverts (corals, crabs, snails, etc.) cannot be treated with copper based products like fish can be. Quarantining everything is only way to ensure no parasites like ich get in your system and even then its not 100%.

After I lost many fish I started QTing all my new fish out of frustration because I would get my tank stocked up with 4 fish or so and add another and get a parasite that would take out all my fish so I went to a 3 to 4 week QT process using cupramine for 2 weeks, and prazipro for 1 week and 1 week of just observation - all 12 fish made it through this process fine and I did it in 2-4 fish "batches".

Now that my tanks are stocked with the fish I want, I run two display tanks and while I do not QT all inverts I do try to dip corals (which will not kill ich but helps with other pests) and when possible I also add them to my smaller tank (has only 1 fish) for 3 to 4 weeks to see if I get an ich outbreak before I add the corals/inverts to my larger system.

It is easier to QT fish, snails, crabs because you dont need a nice light. It is harder to QT all corals as some require high light and very good water parameters which are harder to maintain in a small tank usually with poor biological filtration and that is the reason many people do not QT new corals but dip them and try to add as little/no water from the tank you got them from (only what you cant "shake off" the coral/invert).

Many LFS and people selling corals (even ones on this forum) have ich in their system so its always buyer be aware and always up to you to decide what your appitite for "risk" is, some people live with ich and fish can fight it off if they are happy, eating, not stressed, etc and some go to the extremes to make sure not a drop of outside water or untreated coral or fish enter their main system without going thru a QT process.