May a Western Catholic Hold Eastern Opinions?

Certainly. The tradition of the Church is one. The divisions of East and West are simply questions of culture and different emphases. There is no doctrine held by the universal Church that is purely Eastern or Western. The two traditions are complementary and hardly exhaustive, since even within the Western (or Latin) Church and the Eastern (or Greek) Church there are many different spiritual and theological and liturgical tendencies that show the fullness of the Faith without contradicting it.

When you find someone claiming that a doctrine held definitively by the Church—for example, purgatory (although one does not have to use the term itself) or original sin (as in the case of the dogma of the Immaculate Conception)—is meaningless or nonexistent in the Eastern Church, you may be sure that he has not understood the meaning of the theology that teaches these truths.

That being said, you are free to avoid the Augustinian emphasis on fallen nature in favor of, for example, a sunnier view like that of St. Gregory of Nyssa, or to emphasize the progressive aspect of the intermediate state before heaven rather than its punitive aspect.