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I don't think you have to worry about algae in that section. You can put a piece of thin black plastic against the glass on the top off side if there are any algae issues.
If 7 gal. is enough for your top off that looks good. You can take a few inches from the refugium and make it a little bigger. Either way that should work fine.
I would keep the sand lower then the plastic trim on the bottom of the tank. This way when you clean the glass your not picking up sand and scratching the glass. What do you have in the wet dry filter. Not bio balls I hope. If so I'd replace them with some live rock.
If it's a small sump you can get be with the thin glass from Lowe's. But going to a glass shop for 1/4" glass would be best. It doesn't have to be tempered glass.