I run an 8k on mine, wouldn't run anything smaller.
Multimounts are fine for an occasional winch or multiple vehicles, but they stick out too far and are weak in a side load imho. I've seen mounts with 2 recievers on them, those would be better.
Running your winch underneath would work in a pinch, but if your stuck in mud it wouldn't be very fun getting it through there. Also terrible on your cable making that sharp of a bend at the fairlead.