I wouldn't worry about pellet size and choice too much. There really isn't a massive difference between Geco and Quan Yiang, except that a very few out of every thousand Geco might drop just outside the group. If you are very worried then vice the pistol, and adjust the velocity of the pistol until all pellets are going through the same hole. Steyr and Morini will put 10 shots through a 4.5mm hole...... Steyr at about 160ms Morini at about 150ms. Other pistols I am unsure about, but most info is on the net somewhere.....
-Nick-