I can't seem to set the time limit for various game types
There is a known bug in Halo CE where the custom gametype time limit doesn't save properly and stays as 10 minutes. You can override this setting by using a small Game Type Fix Utility that is available for download.

First create the game type you want with all the weapons, vehicles and attriutes then save the game type. Then open the gametype with the utility and change the time setting to the length you desire.