Are you asking about using the Manticore in a discrimination "mode" which means not using Goldfield or are you thinking of setting up a custom discrimination pattern within Goldfield?
I would not use anything but Goldfield Multi for trying to hit flakes which for me means any small nuggets under a tenth of a gram down to a hundredth of a gram.
Using a custom discrimination pattern in Goldfield Multi works just fine.
Rejecting everything but target IDs 1 to 4 might be a bit nuclear depending on the ground conditions where you are detecting. If the soil is fairly mild without much variation in mineralization, it might work but I would at least accept 0 to 8 or even 10.
Also, check your upper ferrous limit in the 0 to 20 or so range. The factory preset ALL METAL ferrous limit pattern has the entire upper ferrous limit grayed out from 0 to 99. At least in the areas I hunt for small gold, many sub tenth of a gram bits have their 0 to 8 or so target IDs plotting within the preset upper ferrous limit even when they are on the surface. Hot ground will pull those little nuggets into that upper ferrous limit instead of plotting them in the center non ferrous area. If you are hunting with the horseshoe button engaged it's no big deal. If you are using a discrimination pattern and hunting with the horseshoe function turned OFF, those tiny nuggets will have red numbers and fractured audio if they are plotting within the factory preset upper ferrous limit.
I remove the upper ferrous limit from 0 to 20 or so just to be safe. Most of the square nails in the areas I hunt false in the low 20s so they get caught in that unchanged area of the upper ferrous range which helps me identify them as iron. Any nugget that is big enough to have a target ID above 10 or so is probably going to plot mostly in the non-ferrous area of the 2D screen, depending on depth and amount of mineralization.
Just watch out for the possibility of target masking by the ground or by bigger ferrous targets near a small flake. It doesn't take much for them to get completely swallowed up and masked.