I got this right, but for a split second I was about to write "Mina usen hiljainen". I think I do get confused between Mina and Olen... Would it also make sense to say "Mina Olen usein hiljainen"?
Yes, the personal pronoun, except for the 3rd person, is optional in Finnish. But the verb olen must be included.
Is 'olen hiljainen usein' right?
Grammatically yes, just sounds a bit odd. Puts more emphasis on "usein".