In French Polynesia its often hard to find anywhere to anchor less than 30m deep. We have never had a problem with our 30kg WASI anchor and 100m of Aqua Maggi 4 10mm chain. 3:1 was our general rule. In other island groups anchoring can be less challenging.
The bottom can be anything from mud, sand, broken coral. The up side is that generally the wind comes constantly from the SE so as long as you're lined up into wind and have a good hold on something, you should stay there.
You really need all chain in the Pacific. Rope will chafe on coral and eventually let go. Chain will occasionally wrap itself round coral heads, which is a real PITA but it doesn't break.
We have a second anchor in the forward starboard locker but have never used it.
We have a Fortress anchor in the lazarette for a stern anchor which we have used a few times to keep the boat lined up in difficult anchorages.
The idea of putting two anchors on the same rode one behind the other doesn't get much support from the reading I've done...I've never tried it.
Port Havannah, Vanuatu