You can "easily" find a smaller species of mantis, preferably a spearer over a clubber type. They're not that hard to keep, but keep in mind that the more common species, aren't as "colorful" as something like the ones mentioned
Nano Fission Skimmers would work, but of course, there are other options.
In the end, no smaller skimmer is really that "great" but they do an ok job for the size of tank.
10 gallon tank, arg. sand, possibly an hob filter if you prefer it, the skimmer, few pounds of lr, flourescent lighting, and a few other essentials and you should be good to go.
I would suggest looking over nano-reef.com, mantises seem to be big over there, and there's usually quite a few threads about them.