I suppose in theory a 30 litre tank would just about work for a dwarf gourami. Not sure a Biorb is perfect, since these are bottom-dwelling fish rather than midwater fish, so they'd be stuck in the narrow bottom part of the thing most of the time. A regular rectangular tank would be better.
However, and I say this something like 17 times a week, approach dwarf gouramis with extreme caution. Virtually all the stock from Southeast Asia are exposed to the "dwarf gourami disease" and often start to die the moment they leave the fish farm. There may be some specimens that, for one reason or another, do fine. But you wouldn't believe the number of queries I answer about sick dwarf gouramis. I work on a couple of sites "behind the scenes" answering questions, and messages about sick gouramis come in literally every single day. Visit any aquarium shop, and you'll see a sizeable proportion of any specimens for sale showing early signs of dwarf gourami disease: loss of colour, lethargy, sitting on the bottom of the tank, odd pale patches on the body. Eventually the bloody sores kick in, and then the fish dies. Purely on experience, I'd estimate something like 50% of all the dwarf gouramis imported die within a year. They should be living 3-5 years.
So, unless your retailer gives you a copper-bottomed assurance that these fish are either locally bred or else imported from somewhere else, say Florida, then I personally wouldn't touch them with a ten-foot barge pole.
Cheers, Neale