When I started playing with people it was the same kinda thing. Front wedges and turn the amps up. Things have changed drastically with live sound. Having an FRFR and an IEM won't do any harm or cause any loop. What you have the in the IEM is your own personal mix of the whole band, even yourself. Think of it like this, you're taking what floor monitor mix you would have in front of you and putting in an IEM. My last band, we had 2 wedges out front and our drummer had a wedge. The rhythm guitar player used IEMs and he had a FRFR for his modeling unit floorboard. Sometimes our lead player would use IEMs and he too had a FRFR that was by my bass cabinet.
There's no rule that you have to use one or the other or both. Some will use IEMs but have a wedge or two on stand-by in case the IEMs go out and then they can still hear everything when taking them out. Otherwise, you're relying on hearing what is going out the FOH for what you can't hear.
In the worship band I play in, we use IEMs for everyone. No stage wedges at all. The only thing you hear on stage is the drums behind the shield and some acoustic guitar. We have our own mixes in the IEMs.
If you're doing old school stuff, you could easily use a FRFR, even the Headrush 12" would be great, and send a signal to the FOH and if the PA is vocals only, turn it up. Think of it like this, your modeler is like taking your old school pedalboard, amp, mic'd up speaker cabinet, and putting it in to the digital world in a nice compact unit. The FRFR is like the PA that said mic'd cab would run through. That's the simplest concept. And sending the FRFR to the FOH PA is simply an XLR cable connection away.
Depending on the band, a lot of backlines I see now are a banner on either side of the drums with the bands logo or name on them and that sits in front of whatever speakers are behind it. If you need a 4x12 on stage for looks, get one that is unloaded and sit that on the stage and then put the FRFR where you need it.
The whole modeling / FRFR and using with or without a PA is pretty simple. Just take the big stuff and squeeze it down in to an itty bitty living space....lol.