i keep a randalls pistol with a breeding pair of stonogobiops nematodes in a four gallon vase, with a 5 inch sandbed. the vase itself is 9 inches wide and 19 inches deep. the vase and inhabitants have been in there for a couple of years, maybe three..i can't remember. the shrimp itself i bought back in 2009. it's still going strong. the key to keeping the tunnel system and sandbed alive is using an air lift tube that helps circulate the water through the tunnel system. the tunnels themselves are as old as the vase setup and the shrimp is constantly improving it. the ant farm effect is entertaining in itself. watching the shrimp usher out the weekly clutch of baby gobies is kinda cool too.
just as a note, if i had the chance that you have, i'd install a small cup in the sandbed with a camera underneath to photograph the gobie pair and shrimps activities.
i'd be interested in what you end up doing.
picos are far more easier than most aquarists will admit to. although experience does help, i think you could be just as successful. having a larger tank with it's weekly 20-40 gallon water changes are a pita. changing a couple of litres every day or so is a breeze when compared to that.
what size, shape pico are you thinking of using?
like the guys above say, stick to a randalls pistol... they're far less aggressive, they are guaranteed to tunnel and will immediately pair with stonogobiops. and you don't need the gobies, but the effect is much better with them. if you're serious about it, i've got some other pointers that might raise the eyebrows of most of the folks here, but have had great success myself. let us know how it goes for you... there aren't that many pico reefers around here and i'm always lurking for them...