I have been beefing up the fiend presence greatly without any problems. After testing the battles between approx. 12 fiends vs 8 or so NCR Troopers(between Mcarran and the bridges leading to Fiend territory), some of which I added to prevent Fiends from overrunning NCR, had no issues.

Previously to fiddling with GECK, I installed the IWS(Increased Wasteland Spawning Mod). I decided to duplicate some of the Great Khan soldiers that were added in the IWS mod to offset my own added Fiend forces. I also moved some of the Bighorners and Giant Mantises' spawn points further away from my Underground Hideout in Spring Mountain with the intention of later creating a settlement there with RTS mod. Seemingly immediately after, I was unable to Save or Quicksave in and around Fiend territory(despite loading previous saves), and oddly around the Grub and Gulp merchants(started a settlement with RTS mod). I can save in the Spring Mountain Area, the game crashes when I save beyond the mountain pass leading to Violet's trailer camp and the gas station(when my character is close to Chance's map).

So I removed the duplicated soldiers, but the same problem is occurring. I know the problem is my mod, because when I disable it I can save fine in the area.

I am very new to the GECK so I am not sure which bits of info are relevant here, hope I hit the nail straight.