Aroma hops are added at the end of a boil or at "knockout"( when heat is turned off). More than one variety of hops can be used depending on the blend you want to achieve. By adding them at the end of the boil less of the aromatic oils are lost to evaporation. There are no real restrictions on which hops you can use to personalize your beer, just remember that your choice will reflect in the end taste. Dry hopping with these these can also really bring out a more complex taste especially in IPA'S.
The primary use of hops is for bittering. Bittering hops additions are boiled for 45-90 minutes to isomerize the alpha acids; the most common interval being 60 mins. There is some improvement in the isomerization between 45 and 90 minutes (about 5%), but only a small improvement at longer times ( <1%). The aromatic oils of the hops used in the bittering addition(s) tend to boil away, leaving little hop flavor and no aroma. Because of this, high alpha varieties (which commonly have poor aroma characteristics) can be used to provide the bulk of the bitterness without hurting the taste of the beer.
Flavoring hops which are added midway through the boil compromise between the isomerization of the alpha acids and the evaporation of the aromatic oils. This achieves a yielding of characteristic flavors. These flavoring hop additions are added 40-20 minutes before the end of the boil, with the most common time being 30 minutes. Any hop variety may be used. Usually the lower alpha varieties are chosen, although some high alpha varieties can leave pleasant flavors. Often small amounts (1/4-1/2 oz) of several varieties will be combined at this stage to create a more complex character.