Hi all, most of us who have been experimenting and building with BA's have chosen the drivers based on a number of factors. Price is usually the major one when just getting started. All of us have burned up BAs during the soldering process, pulled out solder pads during excessive handling, clogged up drivers by accidently pushing acoustic materials inside of the driver, and there have been the occasion crushing of the metal housing of drivers trying to force them into impossibly small spaces. The more expensive the driver, the less forgiving they tend to be. There is a real need to develop skills with handling miniature components. As far as recommendations about BAs, the only true recommendations that can be made are to build IEMs using the same exact configurations that have proven to yield good/great sounding DIY IEMs in the past. I totally understand the DIY spirit of wanting to give birth to your own unique design and configuration but getting to this level of DIY is usually dependent on your skill as a builder, testing equipment, patience to acoustically tune them and most importantly, your willingness to accept failure. There are numerous pitfalls during the build process that make it challenging for us to help pinpoint what the problem might be if you're using BAs we're not familiar with.
I have no doubt that many of you newcomers have some pretty cool ideas and I look forward to seeing your progress. New ideas definitely spark old flames in some of the veterans on this forum.
I'm not sure I actually answered any of your questions but the best thing you can do is just get started and post regular updates on your progress. Be honest with the challenges you are facing and have the willingness to accept advice that may be counter productive to your design. Many of us have seen well intentioned DIYers join this forum with an excess of excitement only to go dark a few short weeks later. I would venture a guess and say there are probably a dozen of us over the years who have actually managed to produce custom IEMs that we found enjoyable to listen to.
Cheers!