Thighs: this is just an issue with the FX thighs that they're very barrel shaped, and yes we're referring to the gap at the knees. You have to trim at an angle and some folks use a heat gun. If you have some cash, you can always try upgrading to AP.
FX Chest: yes, trim the bottom. I saw TK4205 actually use a mark one eyeball and free hand trimmed the bottom of someone's chest with nothing more than a knife. Others stack a few coins with an exact blade which allows normal humans to track the same curve. Replace: definitely with an AP, as it's smaller and has a more correct sculpt. You should replace the chest/back as a set.
Belt: also replace with AP down the line. It should be fine for awhile, this is only if you want to get anal in proportions.
No doubt getting FX to fit on a smaller person is more of a challenge (the sculpt base was someone 6'+ IIRC), esp. the thighs which are a challenge for almost everyone.
That said Darman, we have utmost faith and confidence in your being able pull it off. Michele will be the first female Elite and you'll be the first married Elite, so being sooooo close with just a few items to go...just think how awesome it will be when you're done!