I use the 4 season fly off my Hilliberg Nallo tent plus painters plastic that you can buy from Home Depot for around $2.00 for a floor all the time in Colo and Wyo from 5,000' elevation up to 14,000'. Once September rolls around most mosquitos and bugs are pretty scarce at higher elevation in Colo. Never a worry about rattlers....especially at 8,500'+ in Colo! Never had a problem with mosquitos, bugs, spiders, mice, or rattlers either. Not to elevate your blood pressure but I would be a lot more concerned about a rogue black bear trying to rob your food or tear up your camp when you are gone than bugs.
Obviously if camped near a bog or water there may be mosquitos? If you do some research you'll likely find that rattlesnakes crawl into their holes most evenings before dark because it's way too cold for them to be out! Take your cloths off at 8,500-9,000' elevation in Colo in Sept and you'll see exactly what I'm talking about! The only problem I see is if a rattler crawled into your tent before night at lower elevation?
To ease your mind a little.....I did a search on Rocky Mountain National Park and couldn't find anything in regard to rattlesnakes or a death from a bite there. I've never heard of a rattler in RNMP? Someone died from a rattlesnake bite on the small mesa above Golden but that is a lot lower elevation than RMNP. There may be exceptions but I have never seen a rattler at over 8,500' in Colorado....and I spend a lot of time in rattlesnake country every day at work.