>I believe originally, naturally fermented sausages required some time to develop the flavor desired. Fermento just speeds this up, <
Well, let me cite an example of this effect: When using nitrite and not desiring any fermentation, one can pretty much mix, grind, and stuff--all in 24 hours (or less), if the grind is fine enough on some sausages (not all). All of that can be done at 38 degrees. The conclusion is, that the nitrite cure does not need bacteria to accomplish its work.
I have an old-tyme summer sausage recipe that dates from the 1930s. It calls for adding all the spices, binders, and cures (nitrate and nitrite) and letting it cure and ferment for 5 or 6 days, slightly above 40 degrees. Below 40 degrees, the natural nitrate to nitrite conversion process and the fermentation go too slowly to be practical. I have done this at the proper temperatures and it produces an excellent sausage, with a mild tang. This method uses the sugar that occurs naturally in the muscle tissue.
If you add Fermento ahead of time, you will get extra Lactic Acid. How much depends on many things and you may or may not like the end product. All Fermento does is to supply needed nutrient to the bactieria to get them off to a rip-roaring start and keep them going.
Personally, I'd try adding half of the Fermento, as the fermentation process is a process that is best described as exponential and half of the stuff would probably get you plenty of Lactic Acid in the end, considering the extra time and that exponential function.