This looks like it would be pretty easy to fix, especially the cross in the back. What have they used to secure them? My assumption is they've used leather rivets that hammer together and look like this
. They're similar to denim/jeans rivets but are longer to account for the thickness of the leather.
So all you'd need to do is cut off the original rivets (from the back, if you don't have any tools use the little cutting section on a pair of pliers to mangle your way in and cut them, or you can drill them out from the back with a drill bit that is a tiny bit larger than the rivet hole), and then shorten the cross straps, recut, punch new holes in the cross straps and rivet it back together.
If the shoulder straps are too long and the holsters are sitting too low on your torso, the quickest way would be to put in some new notches higher in the shoulderstraps and retie the thonging. The holsters themselves will hide the excess and the original holes. If you do this make sure you're careful with your cutting so that the leather doesn't risk tearing, and you might want to consider reinforcing the back of the new notches.
Hope that helped. :/