Thats where it gets tricky. You can mount them under the pendants if there is room. Or you could mount them on either side. You dont have to mount them directly to your canopy. A lot of people mount the end caps on a board and either lay it on top of the tank or you could make extensions from...
Disom - You can use the MH no problem. Except for maybe some heat issues. It depends on where your fuge is located. But dont hesitate to use it, you wont overkill your algae.
How long has the sand bed been in your other tank? You can take the top layer off, and then transfer the remaining sand to your new tank, and then put the top layer back on. This will help keep most of the detritus in the sand. Except for the very bottom layer. I usually leave this because...
First off mount your pendants perpendicular to the length of your tank. The light spreads out the side of the bulbs, not the ends.
As for your canopy just hang the pendants from some chain or hooks if you want your lights closer to the water. You wont have to worry about water splashing as...
