for malawi species you want your ph to be 7.4 or higher. It is far more important to have stable water conditions than trying to adjust conditions because it is far to easy for it to go wrong and swing and hurt or kill the fish you have. I kept my malawians in around 7.5ph and they did very well and my current tangs are in around 7.3ph because i am on tank water.
If you are on town supply water then the ph is usually high (7.6/7.8) or something like that and suitable for africans, if it is i wouldn't bother purposely trying to raise the ph but if you wanted to have the limestone for your aquascaping then go ahead. They will raise the ph, how much i'm not sure but it won't be detrimental to the fish i don't think.
If you are worried about the not being safe due to contamination of sprays or something i would soak them for a few weeks, some may say boil them or something but i wouldn't bother. I would just scrub them clean, rinse them and put them in but i guess that is my risk. I