25-pitch chain will work fine for drive-train applications if you can manage the manufacturing tolerance required, and save you a fair amount of weight over 35-pitch. Team 190 has always used 25-pitch roller chain, and Team 228 will be going with 25-pitch this year.
As for tensioning roller chain, Team 228 always uses an extremely simple method of using HDPE bar stock with an off-center hole. As you rotate the HDPE around the center hole, the diameter changes and works perfectly for adding tension to roller chain. Here's a photo of three applications of these simple chain tensioners from our 2007 robot:
