Feather rock is probably Lace rock, which is another form of Lava Rock.
depending on where they are quarried, some rocks can have limestone.
limestone is a form of calcium carbonate which can rise your pH to 8.0
the easiest way to test for limestone is with an acid like common vinegar.
if poring vinegar on the rocks makes them bubble, they have limestone.
so if the rocks in question pass this vinegar test, you may use them.
unless your fish chase around your tanks fast and often, I would not
worry about the sharp rock edges, as they know to avoid them, and
beside you can always sand down the worst of them before use. I'd
avoid shaping the rocks as that will decrease their natural look, simply
pre-select an assortment of rocks you can arrange the way you like.
take a cardboard box near the size of your tank, lay it with the open
to the side, and arrange the rocks inside the box. that way you can
try lots of ideas without bothering your fish. once you have an idea
you like, rinse as much rock dust off as you can, then place them in
your tank, and don't be so quick to cover them with moss. you might
like them exposed, so save the moss idea for later, or simply jam some
moss between the sides of each rock to start. no need to tie it on.
Here's an article of someone shaping this rock with common tools.
notice it's for cichlids, fish that prefer the water to be high 8.0 pH
so make sure you try the vinegar test before use in a planted tank.
remember to only use Masonry grade drill bits and saw blades on rock.