butterflysaddle, welcome to TE.
I personally wouldn't use Monistat as a preventative or a ride lubricant. Using it frequently will lead to the yeast strains you carry becoming resistant to it and becoming meaner and nastier. So that when you actually have a full blown infection, the over-the-counter treatments will be ineffective.
I'd go with the chamois cream unless you're allergic to any of the ingredients. If you have sensitive skin, consider a personal lube or diaper rash cream. They will cut down on chaffing. In my experience, I have less problems with vaginal infections when there is enough lubrication down there whether it's my own natural stuff or outside help. The other thing is to not use any harsh soaps that strip all the natural lubricating oils on your skin. Maybe try a mild lotion cleanser when you bathe and right after your rides. If a showering facility won't be available at the end of your ride, you could try carrying baby wipes and clean change of clothes to get out of your shorts.
Everything in moderation, including moderation.
2007 Rodriguez Adventure/B72
2009 Masi Soulville Mixte/B18
1997 Trek 820 Step-thru Xtracycle/B17