The Liga Venezuela Beisbol Profesional runs through December 30, so Guillen would be playing for just over a month. If he's playing at first, this does nothing to help him prepare for next year with the Tigers. The Tigers do not have control over the position played in a winter ball league, from what I can tell (I read this somewhere, but couldn't put my finger on it now).
Side note, Freddy Garcia is also on the Magallanes roster, number 60. No appearances yet as far as I can see. The Tigers have had no talks at all with Freddy, so I guess they're waiting to see how he does in winter ball. Other teams have expressed interest according to MLB.com (none specifically named). If he does well, I think the Tigers should try to sign him to a reasonable contract. We've got to get some bargains, and from what we saw of him last year, combined with a good performance in winter ball, I'd say he'd be as a good a gamble as any for the price.