Lost my iPhone 6! Good thing I had find my phone enabled! QUESTION.

If I remote erase everything now, will it render the phone unusable requiring my original passcode to unlock? I

Your phone is already passcode locked, and whoever has the phone needs the passcode to use it and access the data, right? If your erase the phone, there will be no data to access and the phone will have to be reactivated with your Apple ID and passcode just to use it.

Also, the battery indicator is going down so if it dies, it essentially means I'll be unable to remote erase correct?

Correct. Also, if it is turned off.

Also, if I have my original passcode activated, how safe is my data anyway?

Not clear what you mean. If whoever has your phone has your passcode, they have full access to your data and use of the phone.

Read these support articles:

What to do if your iOS device is lost or stolen

Reporting a lost or stolen Apple product