Check that the verticle crosshair remains parallel with the plum line when the gun is level as you tighten the mount screws. The only way to determine which pellet is right is to shoot a variety of pellets through your airgun and see which one shoots the best. Most FT shooter use the Crosman Premiers or Beeman Kodiak Match pellets. The is no definite answer here. A lot has to do with personal preference and the velocity of the airgun you shoot.
Heavy pellets offer more resistance to wind and that is their big advantage.
However you take a big hit on the amount of trajectory you have to deal with. Light pellets are best for lower velocity airguns with lower power scopes. This will give you more error in range finding and still get a hit. If you have a higher power scope and can range find more accurately, you might go with heavy pellets. If you have a higher velocity airgun, use heavier pellets to lower wind deflections.