Will you be using RO/DI water remineralized with a GH+ booster only?
You may be able to find breeders who keep crystals in higher pH water, however you are better off trying to adjust your tank parameters to crystals rather than trying to adjust crystals to your tank parameters if they are not "ideal" for crystals.
You could put a layer of buffering substrate on top *OR* consider a modified hang on breeding box that uses a pump to push water through the box. Fill the breeding box with substrate and this can buffer the water lower pH levels. Whenever the pH starts to rise, just replace with new buffering soil inside the breeding box. The SL-Aqua soil may be ideal here since it releases minute amounts of ammonia. ADA soil would need to be "cycled" in a bucket prior to adding it to the breeding box, otherwise it may release too much ammonia within the tank.