hey clownfish11790,
i'll give you my take on this.
first, if you are serious about wanting SPS here is some links to good info.
SPS FAQ
i am assuming your tank has been setup fairly recently. i would wait until your tank has had some time to establish & stablize...at least 6 mos IMO, if not longer. as for the light, as mentioned, you may be able to keep some montipora if you keep them high in the tank, but the growth and color may not be that good. also, if you want to keep SPS you'll need more flow. 15x turnover (just a guess, i'm not sure exactly what you HOB filter's GPH is like) isn't enough. i would probably get rid of the HOB and opt for more powerheads...i'd look for small powerheads like the micro jets or the tunze mini, and you would want to at least double the turnover you have now.
even if you don't want to keep SPS i would remove all the media from your HOB filter, and just use it for flow, or to run carbon...or if you don't end up adding a below tank sump/ refuge you could convert the HOB filter into a HOB fuge.
to answer your questions about the sump...well, first you need a way to get the water down to the sump (obviously

). you can either have your tank drilled for an internal overflow, which is tough because your tank is already setup...you could buy a second 20gL and have that drilled and then use your current 20g as the sump (not sure how much room you have below your tank). or you could use a HOB overflow. one of the problems you mentioned is only a problem for HOB overflows. with a drilled tank w/ internal overflow there's a drain in the tank and the water flow is controlled by the overflow, but with a HOB overflow there has to be a siphon created to get the water up and over the back of the tank. certain types of HOB overflows have a flaw in the design that can cause the siphon to break, so if the power goes out and the siphon is lost when the power comes back on the display overflows. now, i have no personal experience with HOB overflows, so i'm going largely by what i've read about them in my research on RC in setting up my own tanks. the lifereef overflows get great reviews, and people claim that they never lose siphon after a power outage or something. my father-in-law has used Amiricle overflows on his 125 for 9+ years w/o problems, so those are another HOB i would trust. the CPR overflows are ones that have been known to have problems, but as i said i've never used a HOB overflow so i can't comment on that.
now, your second problem with a sump/ fuge setup that your talking about is actually pretty simple to prevent, and really shouldn't be a problem for anyone who takes the time to set things up properly. if the power goes out, your return pump stops, if that happens you need to have enough room in your sump to hold the water that's in it, the water that's in your drain and return lines, and how much water will drain into the sump until the water level drops below the teeth on the overflow. as long as you put some planning into it, this really should not be a problem. the only other thing is to add siphon breaks to your return outlets if the return outlets are below the water line. this is simple, its just a matter of drilling a little hole in the return just below the water line of your display. if the return pump is shut off, when the water falls back down to the sump it creates a siphon. the holes allow air into the line once the water level drops below them, and this breaks the siphon.
whew...typing that all out makes it seem a lot more complicated then it really is.
