mangrove tanks are a great way to filter nutrients out of your main display.
People usually choose them when they want to have a sump out in the open
that looks nice.
1) with a 20 gallon there is always the risk of cracking because the glass
is pretty thin. You will have to be very careful and chances are still good
it will crack. Try to do it on the back so you can just seal up the crack
and the mangroves will cover it up.
2) It would be a lot easier to but the mangrove tank below the main and then
have a normal pump go from the mangrove tank back to the main. Either way
the pumps in the biowheels are not very powerful and are not meant for that
purpose. Better off just getting a small pump. They also come with a guide
on the box telling you what flow you will get depending on the height it is
3) I would just use dry sand. It will become live over time. Especially
since it will be connected to a mature main display. Undergravel filters
will get clogged and are a bad idea. Get a few nasarrius snails. Between the
snails and the other micro life in the sand they will keep it aerated
4) Open top sounds good. The heat from the lights will probably be good
enough to keep them at the right temperature I would guess.
Roam around this site for a while. He has a lot of good info on mangroves
and natural filtration.