I'd take them back and look into some of the really cool nano stuff out there. 5 gallons is just going to be tough to keep everything in check, even for someone with experience. Nitrates will jump up quick. Salinity can increase if water levels drop. Temperature can bounce around based on room temps and you dont want any more than a 2 degree change in a day. It's just tough to maintain a healthy saltwater environment in such a tiny space. I think 30-40 is good for starters, but I would not be comfortable going much smaller, even with my several years of experience.
Reefs are great, but you would have to really watch your parameters closely. That is a delicate ecosystem that is not always tolerant of big fluctuations, which are very likely to occur in a 5g tank.