No, I don't add limestone but only because I don't have any handy. Do you get yours from a hardware store? I have rocks other than hydroton like pea gravel and also include some large perlite. I have added dolomite before but like you say it washes out. In the past, moss was my primary medium for flask seedlings but in the past I was growing seedlings either in a greenhouse or in house. Now, I'm putting seedlings in the basement and have killed many when using moss. Mostly paphs and some less water loving phrags but I just switched to improve mortality. Also having luck with seedlings with orchiata mixed with bonsai pumice and perlite.
I have some smaller 735s so we are probably pretty similar overall. I think I remember you posting that you got yours in September 2019 which is when I got mine. Ken