Question Adding battery to existing Enphase IQ system

I have 12Kw grid tied system with an Enphase combiner.  I've been looking at how to add a house battery and unlike everyone else I want to do it as frugally as possible (sarcasm).  Once I understood what the Gridboss does (Will Prowse videos helped a lot) I got excited about it.  I think I can replace many boxes outside my house where the meter is.  Here are the questions I have:
1)  Inverter. I feel like the Flexboss might be overkill for my usage.  But in a one of Kelly and James' videos, James mentioned attaching the PV AC to I think the load connector on the flex box.  I didn't understand this.  

I want a right sized inverter (and I'm still trying to get my usage stats) and I'd like the PV to keep powering the house/charging batteries.  I'm less interested in selling power back as it's pennies on the dollar.  

Can someone point me to info on how to configure such a system?  Either Flex, 12000 or 6000, and the Gridboss.  Do I even need the Gridboss?