Galaxy S7 user with inactive iPhone 6 - Can I get iMessage with my active phone number?
Once you start using the iPhone on your account, and activate iMessage, you will no longer be able to receive SMS on the Galaxy from any contacts who use iPhones, because from Apple's perspective, your number will now be an iMessage number.

If you wait until you're ready to leave to do this, that will work. However, if you pause/cancel/whatever your service with Sprint, you will only be able to receive iMessages if the person has the email address associated with your Apple ID in your contact on their iPhone, and you have that email address set up for receiving in iMessage settings on your iPhone. If your service stays active, messages from other iPhone users will still be received by your phone via your phone number, but only if you're connected to Wi-Fi (or enable data roaming, which would probably be a bad idea if you like keeping your money).

Once you get back, assuming you want to resume using the Galaxy, you'll want to deregister your number from iMessage or you will back in the same predicament as far as not being able to trade messages with iPhone users.