RMNP is a popular place and you'll probably get differing opinions. Do you want to fish inside the park or just in the area?
In the area, the Big Thompson is a good option and the private water is fun to fish. I've used Kirk's a couple of times and I've had a great guide and a less great guide from them. If you decide to use Kirk's, I'd get the name of the guy Woollybugger used and request him. (Don't let any guide take you below the dam in EP, you can do that yourself.)
If you want to fish inside the park and this will be your first visit, I would get a guide. The park is huge and it takes time to figure out what is what and where to fish. Moraine Park and the trail to Fern Lake have good areas to fish, but both can get crowded.
We drove through the west part of RMNP this summer for the first time. If I ever go back, I plan to go to this part of the park to fish the Colorado River. Smaller crowds and beautiful water.
If you do hire a guide, be sure to discuss what is included and when the half day starts and ends. I would hate to find out the clock starts at their shop and includes an hour drive to the fishing location.
Have fun! It is a great place to visit.
(Also, black ants were active while we were there last month and the green back cut throat were nailing black ant flies.