You can do what you wish! Some hops are milder in flavor than others, and some are more flavorful, but there are lots of beers that may use 100% of one hop variety for all the additons.
All the hops have an AA rating. the higher the number the more bitter the beer. Also depends on the schedule. Usually use the higher ratings for the start of the boil, then the middle additions are for flavoring, flame out and dry hop for aroma.
The AA rating is the amount of alpha acids in the hops. Depending on the point in the boil you add them you'll extract more ( isomerize ) this component contributing to more buttering. Later additions isomerize less.
Some hops contribute more to flavor. & aroma and are added later in the boil, say after 30 minutes.
Dry hopping adds no IBU's but does add aroma components.