Well i only work with the right handed system but yes the matrices for both systems will be a little different. I remember reading something about the rotation matrix and how it looks different for RH and LH. I think it was on flipcode or gamedev.net. One of those two sites i read this on. It’s not likes its a HUGE diffrence though. Just a sign difference pretty much.

Yes you’ll have to convert it first. Once you do, your matrix library should work fine. Also both post and pre-multiplication are OK,depending on what your goal is. Both have meaning. If A and B are transform matrices and v is a column vector, then

(A * B) * v, means first apply B to v then A
(B * A) * v , means first apply A to v then B