Can I run the app gaining traction on a purchased Dyno and the rest 5 apps on the free Dyno?
Also the billing, for a single Dyno will it be exactly $7 or vary with the use?
You can upgrade only one of your apps to paid dynos, and keep all the other apps on free dynos. Your monthly quota of free dyno hours is per your heroku account, not per an individual app. So your 5 low-traction apps will simply share your 1000 free dyno hours per month. (Note that your quota will be 1000 free dyno hours per month, since you definitely will have verified your account in order to upgrade your high-traction app to paid dynos).
As for the exact charge: If your high traction app uses only a single Hobby Dyno and NO other non-free-tier add-ons, then your bill will be exactly $7 / month, and will not vary with use.