One thing for sure, battery on s6 mini will be smaller than usual. 2300 or 2400 mAh maybe.
So it is better to use power efficient chips. Qualcomm midrange chips are bad now.
They use the old 28nm process. Like sd615, sd410 etc.
If the phone performs nicely, then there is no need for custom roms. Root and recovery
will be good enough.