Usually, if the door is a hollow-core door, you can only take off an inch, maybe a little more. If the door is a SOLID door, then it won't matter.
If you need to trim the door, trim it off the BOTTOM; not the top. That way, the 'profile' of all the doors, whether they're in the same room or not, look the same. THAT is the profile you see first, and most.
Also, most bi-fold doors come with a top track and a bottom pivot. I contacted Stanley Hardware and ordered BOTTOM tracking too. The doors wear much better when there are 2 tracks. (Less stress.) If you go this route, the FATTER track goes on the bottom. It's meant to 'accomodate' carpeting right up to it. If you put the 1/2" track on the bottom (instead of the 1" track), your doors won't open. True Value Hardware, maybe even the Home Centers, can help you with getting the dual strip tracking. Heck, the doors might even come w/it.
You did mention odd-sized opening. Measure each of them, top to bottom, and take them w/you to the store. You might have to trim more than you want. You'll want about 3/8" of space up against the end-jam. (If you use 2 bi-fold doors, like we have at home, that's 3/8" on EACH end.) If you don't have the 3/8" gap, the door won't open/close. My best to ya and hope this helps.