Those tails should work just fine, assuming they have sufficient stiffness for the task. If you're going for an energetic glider (quick climb up, slow float down), go for it, just watch for flutter if you have to dive out of a thermal. In this case, drag from fat tails is only an issue if your powersystem is undersized for the launch -- at high speeds it's bad, at low it'll do.
If you're going for a warm/hotliner . . . I'd say go ahead and get her flying, but don't push her hard until you've replaced them with something better suited. Speed is fun, but flutter kills.
Speaking of painting (and well, another "next time"), yet another advantage of using mylars in the layup -- ever wonder how they get all those pretty colors on composite wings? If you give the mylars a *LIGHT* coat or two of rattle-can paint before you build the layup on top of them, the paint will melt into the epoxy as it sets, giving you a polished, painted look right out of the bag.
If you choose to build out stiffer composite tails (you will, eventually), I'd HIGHLY recommend hinge tape in the layup, and paint on the layup.
For tails, because they are so thin, you can also get away with a variant of "Wax Paper Bagging" and delay the purchase of a vacuum pump. Effectively you make your layup, then instead of a vacuum bag, you place it between LARGE stacks of paper towels, then compress the lot. The squish in the towels will force the outer layers of your layup to conform tightly to your core. The original post I learned from has been 404'ed to the ravages of time, but nothing is ever truly deleted:
https://web.archive.org/web/20060424032518/http://www.sjsf.org/waxpaper/waxpaper.htm
From my lessons learned, wax paper isn't ideal -- it needs to be an impervious layer, thin enough to conform, which won't chemically bond to the epoxy . . . but the smoother the better. I've had fair luck with some acetate/cellophane wrapping papers (bonus, you can get the epoxy to steal their paint if you pick the right side), and saran-wrap (Kitchen cling wrap won't stick or bond, but it's a pain to get all the wrinkles out). Kitchen parchment works too, but it will leave a fine mat finish instead of high gloss most plastics will do. If you go with wax paper, make sure you do the layup on the waxed side.
If you're not certain if a plastic sheeting you have is suited to the task, dribble a bit of epoxy on the plastic, then let it cure. If the blob peals easily off the plastic once fully set, it'll do the same in your final piece. if it *almost* peals off easy, you can hedge your bets by waxing the plastic with neutral shoe polish (or molding wax, if you want the fancy stuff).