I'm not very familiar with the fish you will be keeping or what water params they require. Loaches and GKF need lower pH's right? You'd want to use rocks that won't affect your pH at all. Granite, quartz, slate and some sandstones and igneous rocks are non-reactive--but still test any rocks you want to use.
You can test a rock's carbonate content by dropping vinegar on them--if it bubbles or fizzes it indicates the presence of alkaline compounds--so don't use it (unless you are keeping fish that need high pH and KH)
You could also check what a rock does to the water by filling a bucket with dechlorinated tap water, aerating it overnight, testing it's pH and KH, then dropping the rocks in, come back a few days later and test the water again.
Also, If you need driftwood--I sell it. Let me know if you want some.