My house is about 100 yrs old and has the original windows. Someone recently gave me several new replacement windows from a project that they had changed their mind on. These windows would not fit in my jambs and sills, but would fit in the rough openings(RO).
Do I have to change the RO size and put in jambs and sills to use these windows? Or can I just install them in the RO using self-adhered flashing and wood trim/metal flashing/caulk on the outside?
Some of the jambs and sills are rotted and would need to be replaced even if I was going to use replacement windows of the proper size.

 

Not being able to see your window opening, I would say you could do this, but you have to be very careful of where water will hit, run, enter and exit. You will have to have a sill to carry the water away from your opening and the siding below it. Attaching the windows won't be a problem since it only takes two screws in the sides, but you may have to install brick molding, jamb extensions to the interior wall surface and finally case molding to dress up the inside. Check out the problems you can see with water infiltration, first. Since you say you will have to replace some of the sills anyway, you could start by pulling one of the windows, carefully, and trying to install one of the windows. If it looks viable, keep going. If it doesn't , you have to replace the sill anyway, so you can reinstall the original window and not have lost anything.