Playing Bluegrass on guitar--whether lead or accompaniment--requires more from the strings than if you are just playing and singing solo.   That is because you're in the midst of a banjo, mandolin, fiddle and maybe bass also, so it's important that your guitar have two important qualities:  volume and clarity.  
       In looking at the various strings on eBay, nobody offers (yet ) the ultimate Bluegrass guitar string set.   Worse, when one looks at guitars for sale on eBay, the Seller often proudly exclaims, "I've just put a new set of Elixir ultra-lights on this guitar!"   Friends, Elixirs are not good strings; they do not last long and they are slippery.  So avoid Elixirs and all coated strings.   Second, ultra-lights will barely make the soundboard of the guitar  resonate at all.   Medium gauge is the way to go.  But no heavier.
        Companies such as Martin and D'Addario sell strings in "Bluegrass" gauges, which are usually:  12, 16, 26, 36, 46, 56. in either 80/20 bronze (actually brass) or phosphor bronze.  These sets are essentially medium gauge with light gauge trebels.   Nice strings, but not for Bluegrass!
       Tony Rice (one of the best Bluegrass guitar players) uses D'Aquisto nickel-plated steel in these gauges: 13, 17, 26, 35, 45, 57.   Tony's on the right track, since nickel-plated steel has the right amount of pop and clarity.  Only trouble is the gauges, especially the low E string, which never  sounds good in nickle-plated steel.   So, for the ultimate Bluegrass guitar set, you will need to get two sets of strings:  D'Addario EJ12 medium 80/20 bronze, and a set of  D'Addario EXL 110 nickel-plated steel  (which are electric guitar strings:  10, 13,  17, 26, 36, 46) .   Your  top two strings will be 13 and 17 (you can take your pick from the sets - they are plain steel).   Next, use the EXL,   26 (G), 36 (D) and 46 (A);  then the 80/20 Bronze for the low E string (56).    This will give you a terrific Bluegrass sound.
       I mentiond D'Addario as an example, but other string companies make both acoustic (bronze) and electric (either pure nickel or nickle--plated steel; avoid the pure nickle!)  The companies are DR, GHS,  Sfarzo, Martin/Darco and Ernie Ball/Earthwood.   There are others, but still with name brands and you'll have a really great sounding instrument.
       Good luck!