I don't know what you mean exactly by "sum the slots".
Would you expect that if you merge ten 1 inventory size chests from VerySmallChests that you would get one 10 inventory size merged chest?
Yes, thats it. Just want to make an optimized loading station with this mod, and discovered that I can't store more then 1 stack of ore. There is a setting that can make 3 inventory size chests instead of 1, but it won't completely solve the problem.