first, i am not sure stevens sees olynyk, sully, or faverani as either center or pf. all of them can play either position depending upon celtic combinations and individual match ups. i think stevens can "play" with these two positions and gain advantages against other teams.
nor do i think that the two positions are discrete anymore in the nba. given that there are few "true centers" anymore, a number of players slide between the two positions.
having said that, ainge has already said that olynyk is best served as a pf who can stretch the floor. given his build, rebounding, defense, passing, and shooting ainge may be right. olynyk can bring more to the team by creating spacing for guards etc to cut to the basket and by shooting 3 pointers. if anything, he is more a pf type. i would not call him a center in the nba simply because he is 7 foot.
for sully, he is trying to develop an outside shot that is more consistent, but his strengths are the opposite of olynyk and therefore complement olynyk's game, at least to me.
sully is strong under the basket, rebounds well, and can get put back shots against many nba centers. he is made to work under the basket with his game. therefore, he seems to fit the classic idea of center better, though he is not the ideal height.
faverani, he can play center it seems, but being a pf is not out of the question. he seems to have an outside shot, though not TOO outside.

but then, this is all based upon a few preseason games, so i might be dead wrong about him.
given the talents and size and strength of each player, stevens would be better off having them all play differently depending upon opponent match ups and which of them are on the floor for the celtics.
personally, i want to see sully and olynyk together more. it doesnt matter which one is called a center or pf to me.