A typical 5 mm suit is not that expensive, so a little diy is not that dangerous. The first thing to realize is that if you really have to chop off 6 inches, you are going to end up with very loose, floppy and leaky ankles and presumably wrists. You may have to make longitudinal cut and reduce the circumference of the ankle and wrist.
So my advice is to cut of 2-3 inches and see how a few wrinkles in the legs feel. You may even be able to fold the extra cuff length over the outside and be done with it. This will help to keep the cuff a little tighter and reduce pumping of water
If you can avoid making longitudinal cuts you will also avoid reducing the structural ntegrity of the suit. I’ve done this a bunch.
You definitely want to buy a tube of wetsuit glue to tack down any threads that you cut and if you make longitudinal tapering cuts then you need glue to reconstruct. Another cheap and easy thing to do is buy a foot or two of iron on seam tape and iron on over any cut or reconstructed seams.