Hi Lite can ball up if put in hot plastic, glitters are ok as long as they are not super tiny in grain size.
I mix everything in the raw plastic for cooking except the purple and blue glitters. When the color is cooked and if I feel I need to add more hilite I make sure the plastic will be fluid enough to mix a small amount in and make sure it gets mixed in well.
Everyone finds their own niche in mixing and working with the plastic and in many instances there is not right or wrong way, only what each of us prefers. Like Nick said, the additives mentioned are pretty inert except for maybe adding too much or not enough.