Small gas engine with poor FMIC

[rotrex]

"I have a lowest flow FAV, so I think the hardware is there, but wondering if the HFS-3/4/6 can run at low flow, but only failsafe when the big nozzle and 4 DP nozzles are flowing?"

I don't understand that statement?

For say 200HP and less, 4x 0,3mm nozzles right behind the injectors (from engine perspective) is likely all you need for "normal" WOT operation. Doubling this flow with a 5th bigger nozzle is only useful if you run a lot of boost, say 15PSI and higher.
Dual stage injection is not really needed in your case IMHO. The FAV will take care of the flow control.