I've been tryin to figure out how to make a bigger sump/refugium as well. However, I can help you out with some of this...
Only if you're going to use sufficient lighting
Lots of people use refugiums for holding coral cuttings, etc, but most of the ones I saw for that use were hang on the tank kinds in tanks with mh
pendants so they got the same light as the rest of the tank.
Can I keep fish in there ?
Not advised. Kinda defeats the point, to have an area safe for pods
and the such to grow, and the kinds of fish you can keep in ten gallons are pretty limited, better to keep the fish in the main tank. In my understanding, one of the benefits of fuges is they increase your water-supply/capacity so I would guess (somebody correct me if I'm wrong) that it would also increase your fish/bio-load capacity in the main tank.
can i keep a coral banded shrimp in a ten gallon?
You could, if it were an established area... it would eat as many pods
as it could catch, again, kinda disrupting that aspect of a refugium, but if you had say, shrimp eating fish in your tank, it might not be out of line to keep one in the refugium. Coral bandeds get pretty good sized for 10 gal
though, you probably wouldn't want to have more than one.
Most of your LFS
will have at least one type of macroalgae. It's better to stay away from caulerpa, since when it goes sexual it releases toxins, I think you could find a list of safe macros around here, or I'm sure some other member knows of some.