They said that it would kill the
sand and create toxic matter

I have yet to read this, but I'm 6 months into this too as a newbie. The
reasons I heard for putting rocks in first was for stability. If you get a
sand burrowing animal go under one of your rocks, they could all topple.

On the otherhand I think I do remember reading something about 'dead' zones in
the sand where sulfur and other bad things can build up. Not sure if this was
under people's rocks or just in deep parts of the sand.

I added my sand first before rocks because I didnt know any different at the
time Instead I just pushed the sand out of the way before setting down the
rock, then twist it back and forth several times so it settles.
~John