Ray Allen has played a lot of minutes at the 3 since he came to Boston.
No he didn't, not really. As per 82games, he played 59% of the available SG minutes last year, and only 3% of the SF minutes. Similar breakdown the year before; 67% of the SG minutes, 4% of the SF minutes (and 1% of the PG minutes).
The third-string SF will be Courtney Lee, who's had a similar lack of experience at the SF position but who can probably get by against most teams.
As to Christmas/Joseph, it'll be a fair competition for one roster spot. I doubt a $250k guarantee will make much difference in the end. If Kris Joseph shows that he's the better prospect and especially if he shows he could be a depth-SF who could hold his own if Pierce/Green got hurt. But if Christmas shows that he's better, Joseph's status as a draft pick isn't going to save him.