In most cases, one can acceptably use geniuses as the plural. In fact, when referring to the quality of genius (as opposed to an individual possessing certain qualities or a certain nature), geniuses is the plural that one should normally use. However, when referring to an individual as a genius, the most correct plural is genii (that's three syllables). This is because genius is derived from Latin. It is the first-person masculine singular form of the word. The first-person masculine plural is genii.