I donot have personal experience with any of the three sands linked in post #1. But members in the UK have mentioned the Argos being OK in aquaria. Maybe some other UK members know something about the other two as well.
Do not mix any other substrate with the sand. Substrates will mix due to normal thermal water currents and some fish will dig and mix them up. Basic play sand is probably the best substrate because plants will grow very well in it (it is after all what they grow in naturally, sand and mud), substrate fish have no issues (assuming the sand is not rough and play sand should be OK here), and it is inert so no water chemistry issues.
Areas under wood and rock can compact, but so will gravel, and this is a necessary part of a healthy substrate, having anaerobic and aerobic areas.
Do not mix any other substrate with the sand. Substrates will mix due to normal thermal water currents and some fish will dig and mix them up. Basic play sand is probably the best substrate because plants will grow very well in it (it is after all what they grow in naturally, sand and mud), substrate fish have no issues (assuming the sand is not rough and play sand should be OK here), and it is inert so no water chemistry issues.
Areas under wood and rock can compact, but so will gravel, and this is a necessary part of a healthy substrate, having anaerobic and aerobic areas.