Like I said mine tore up java fern despite it's bad taste. I also tried hornwort and water lettuce since it grew so quickly but the mess was ridiculous.
Either rock should be fine. If you want to buffer your water PH then limestone should be fine but of you're like me and have 8.0 out of the tap, just about any rock will be fine. I use granite.
It's fine since most likely you will be housing tank bred fish that can easily adapt. I personally like the idea of replicating malawi's natural habitat so I would buffer it up to 8, but they will be just fine at 7.6.