not really keeping the cam on freddy means she cant ever kill you even when its down but thats about it, UNLESS you wanna make her take a movement backwards on purpose, but idk when thats ever worth it
as for foxy she gets a movement opportunity every 8-10s (if she's on 20ai) and if any cam is open and being a actively watched she will fail any movements she has, so yes this means if you time it right none of the other cams ever has a use as foxy gets stalled by ANY cam and legit can be made to never move the entire night