Do we have the ability to not enable ARM on a new subsidiary?

no, it is a system wide setting

you can create dedicated items for that one subsidiary and make them "direct revenue posting", which sidesteps ARM completely. But if you want to use the same items, see Karina's answer above

i'll also note that i don't recommend going with "direct revenue posting" because it's a pain to unwind (you can't make a drp item later flow through ARM). However, you could use the Revenue Treatments feature so that all items, when used in the new subsidiary, use a rev rec rule and create revenue plans on value which causes them to recognize immediately on invoicing (i.e the same outcome as drp, but with the ability to change in future).