...When I'm ready to fully stock my tank, as long as I do it slowly it should allow the bacteria time to adjust to a heavier load, right? How do you do that with schooling fish? Won't they be stressed and lonely if you buy just a couple at a time?...
Once you have a fully cycled tank, it doesn't take long for the bacteria to grow to required levels. Case in point, I made the mistake of rinsing my filters in tap water and the chlorine killed off a good hunk of bacteria. The result, a mini-cycle. Ammonia levels increased followed by nitrite levels increased and I had to do a few PWC to keep the levels down. But within 5 days, the bacteria that remained on the filter (I didn't kill it all off) had repopulated the filter to handle the bio-load.
So if you want to stock schooling fish, you can put the whole school in there all at once. But you need to daily monitor your ammonia and nitrite levels and do some extra PWC as called for. Within a week, your bacteria will have most likely multiplied enough to handle the new load. From the stand point of getting your filter ready for the bigger load, I would think it would be better (as appropriate) to first stock some of the non-schooling fish you plan to add and give your filter time to adjust to the new bio-load. Then add the schooling fish. That way, from a percentage point of view, you're not increasing the bio-load quite as much (the idea being that if a set of schooling fish will double the bio-load, add a few other fish first so that when the schooling fish are added you are only increasing the bio-load by 50% rather than 100%.
Now the other thing you could do is add some ammonia to the tank to help build up the bio-load for several days before adding the schooling fish. Obviously you would have to be extreamly careful doing this to insure ammonia levels don't get too high. But I'm talking about levels equal to one drop in a ten gallon tank twice per day (and obviously you migh want to pre-dilute the ammonia in a cup of tank water before adding it more make sure you drop it directly in the tank at the output of your filter or power head to insure it gets distributed as fast as possible).
Now I'll admit, all of my advice is coming for logic based on what I know about fish keeping and science in general. The closest I've come to doing anything I'm talking about in practice is that I'm currently "feeding" a fishless quarintine tank ammonia to keep the filter cycled, but I'm doing so in low enough doses that the MTS and nerite snail (both of which like good water parameters) are no disturbed and start trying to climb out of the water.