i actually think this is pretty nice, much better than gallon per inch rule. For example if you add a bala shark to a 15 gallon, even though your stocking level will be at 60% you get warned because bala shark is not suitable for that small a tank.
And if you add 25 cardinal tetra to a 15 gallon, you get warned your stocking level is 115% but not warned on the tank size as tetra is suitable for 15 gallon.
I would even go 1 step further and say this is great, whoever did it has done a remarkably good job. Adding aggression to individual fish is not needed, just a indicator xyz fish is aggressive/community/schooling etc.. will be fine.
once this program has gone to production, can we add a link to it from here, so any newbie can click and get some idea. There are at least a few threads every week on "how many fish can i add"