For sure a sump and refugium is worth it !!! Algae will grow in the refugium instead of in the DT.
Every skimmer is rated to work under a certain water level. If 10" is too much, then you can lift the skimmer on a support, wood blocks, etc... Be imaginative, but just to tell you it can be lifted if too much water in the sump.
For sure, yes, you can plug multiple tanks on the same setup, don't overstock and under filter. Make sure bio filtration is enough for the overall setup. If you add a 20g to the setup, then add the required amount of LR
The inconvenient of doing this, is that if one of your tanks get contaminated by a ****, then all could be...