Do they need a rock underneath them? The Favites will encrust, so it will need something under it to grow on that is not sand (whether it be glass, rock, more frag plugs, etc). The lobo will create it's own skeleton and can be placed on top of the sand. This does not need to be on a rock, but you can also place it on a rock if you please.
Can they be place directly on the sand and does the skeleton need to be above the sand (Lobo especially)? See above. The skeleton should not be buried, but can be tucked in for stability if that makes sense.
Do they build they're own skeleton and will they therefore just spread out along the sand? Yes, Yes. Again, personal preference to you. Either on the sandbed itself, or on a flat-ish rock, on the sandbed. This is a "set it and forget it" type of coral. Growth is incredibly slow in our systems, and can take upwards of 10-15 years for maturity in the wild.
Bowerbanki Acan
When it's fully covered the rock it's on, do I need to add more rock for it to expand onto? I cannot confirm, but I think can do either/or. I would place it onto a larger rock because I prefer the uniform flat look (growth on a rock) rather than the dome look (growth on it's one). Acan's are stony based corals, so they will grow as they grow and create their own skeleton - think frogspawn for instance. Their skeletons and branches grow on their own. Having the coral placed on a rock possibly allows it to spread and build more structure on top of the rock itself using it as a backbone.
Does it build its own skeleton off the rock, or just expand long the rock? From my understanding, their skeletons are grown from minerals in your water column, then fabricated on top of the rock and tissue grows throughout them. They will do either or dependent on their surroundings (see above answer)
We are in the same boat as you are - navigating the world of LPS. We are fortunate to have a LFS owner who comes each month to do some of the maintenance for us.
We had our Lobo kind of sitting/lying in the sand and he pointed out that the sand had rubbed a “raw” spot which can lead to infection. I’ve attached a photo but it is hard to see now. He advised that the flesh of corals should not lay in the sand due to abrasion.
We mounted the Lobo base on a flat rock with epoxy and set the rock on the sand. I wish now we had shaped the epoxy with a wedge so the Lobo is at a bit of an angle. This was our first try with epoxy so still learning the tricks.
