I think they can.
Ordinary guerrilla operations can isolate Syrian regime forces in strongholds which the regime will find difficult to supply. The regime military seems to already be short of manpower and can’t hold a lot of critical posts. At some point the regime will run out of money.
FSA did manage to drop a Syrian Army helicopter the other day. But these can be replaced quickly. I see much more blood shed ahead. Death toll rising to 30K soon...
Syria has 23M people, about 16M of them Sunni. Iran has 79M people. Iran could exterminate all the Sunnis in Syria and repopulate Syria with their own people if they really wanted to. Or just kill all the Sunni men and take the women.