When To Add More Fish?

Since you finished cycling only very recently, you should be OK to get another 6 now. If you wait too long, the bacteria will die off, or become dormant and take a few days to wake up again. You would then have to add fish as for a fish-in cycle, that is only a few at a time.
If you wait 10 days, you would have to add only a third of the fish in there now, ie 2 more rather than 6. Get more fish now!

Keep an eye on your ammonia and nitrite after getting the new fish. If you want to get more a couple of days later, that should be OK. But longer than that and you'd need to add them in small batches. The guideline is no more than a third of the fish already in the tank at any one go. If you get 6 more fish now (before the bacteria start to die or become dormant) you'll have 12 fish. If you wait more than a couple of days or so, the next addition can only be equal to 4 of the fish you'll have by then. That means 4 fish the same size, 2 fish twice as big or 8 fish half the size of the fish already there.