I think the sizes of flies that you got are pretty ideal for catching trout most anywhere. I wouldn't go as far to say that 18's are large, but they really probably are middle of the road when it comes to what trout really eat.
I use one of these tools when tying on flies size 16 or smaller and find that it works really well. http://www2.llbean.com/product/36013.htm
I would get the midge size, as it will hold flies up to size 12 or so, but the regular one won't hold the 20's and 22's you'll eventually come around to using.
Also make sure to have hemostats with you when fishing flies that size, as there may not be enough of it sticking out for you to grab hold of to remove it from the fish.