So the $9.99 data package with the Reality gives you 25 megabytes of download usage and unlimited access to your email as I'm sure you know.
The 25 mb of download usage is used whenever browsing the web, downloading an app, ringtone, wallpaper, anything like that. Normally when downloading a ringtone you'd be charged the $1.99 for the ringtone and THEN charge for the data usage also. Its just a new way the carriers have started charging for data and its kinda ridiculous.
But since you have the data package, whenever downloading anything you wont get that extra data fee, unless you go over your limit.
The best way to keep track of your data is to either sign up for My Verizon at www.verizonwireless.com, its a free service to just monitor your usage and be able to add/remove any features for your account. You can also call #DATA, (#3282) and you will get a free text message sent to you with your current data usage.
I'm actually not sure if emailing yourself ringtones will work, it depends on if the phone is able to download attachments which I haven't tried out myself on the Reality, but the Reality should come with a data cable to connect to your computer (part of your charger) and you could easily just transfer the ringtone from your computer to your phone that way and not have to worry about using up any data usage on the download.
You can use any apps that you could regularly, navigator is a regular $9.99/mo and that fee will still be there even with your data package. But any app that doesn't say a monthly charge will be added you can use, and the apps will only use part of your 25 mb data package if they say data charge may incur. In that case, using the app will start using up some of your download usage but for a normal user w/o a data package theyd have to basically be paying for using the app while for you the usage is included in your data package.
I hope that helped and wasn't TOO confusing. Let me know if you have any other questions!