Like jslo said, it's a big park. Talk to Kirk's Fly Shop, Scot's Sporting Goods, or Estes Anglers to get some advice. It will be especially important this year because the Fern Lake Fire burned a section of the Big Thompson and its tributaries in the middle and lower sections of Moraine Park. I haven't heard what the situation is in that part of the park and you'll want to check on that.
Also, keep an eye on the runoff: Big T. Flows in the park
. The runoff is still going strong and may be heading up again, and that may impact where you want to fish. Some of the small streams get blown out during runoff and the Big T. in the park gets deep and fast and that will change your fly selection.
Even during runoff I don't think you'll need that 6 weight, unless you want to cover water on a lake. I'd use the 4 weight mostly and keep the 2 weight around for any tiny creeks. As for flies, check with the shops but I always carry Elk Hair Caddis, Royal Wulffs, Humpies, Adams/BWOs, ants, and beetles. Hoppers are nice to have too. But during high water I also carry a selection of nymphs. If you are going to fish the Big T. below Olympus Dam (about 20 minutes from where you are camping), a great alternative if fishing in the park is tough, you'll want some tiny, midgy nymphs too.