I don't really see an issue with the vote being biased. There's no bias in those being forumer of the day, and so no bias in them being eligible to be voted for.
Only thing I would say is that, since you want to do a Forumer of the Month thing to spice it up and add variety, monthly polls might be a bit too slow. After all, we've got 47 people total, yet only 2 months.
I think it'd be more interesting if you did "Forumer of the Week".
You choose your random daily forumers from Monday to Sunday.
Then on the next Monday, you put all 7 of them in the poll.
Then when Sunday comes around again, you declare a winner for the week.
And on Monday, you put the 7 people from the previous week in the poll, and the winner of the last poll is Forumer of the week for this week, until Sunday.
Then, Rinse and Repeat.