Send me a pic of your Rasaboras! How much? Have you had them long? Are they hardy and easy or sensitive?
Sorry I don't do Brit/Euro measurements w/o deep thinking. And I have only had one cup of coffee so far. Is 34l about 18 gallons?
These guys are really tiny. Get 25 hastatus if the Rasaboras are the size of harlequins. I think 25 hastatus would be good alone in a 10 usg. Now really I have had pygmies and not hastatus, so you must wait for more experienced advice.
Edit: Well the experts just had a quick read and passed us by. So we must wait some more.
34l is just under 9 US gallons. (type into google "34 litres in gallons" 8))
You know you want more Galaxy Rasboras I'd think you could comfortably have about 8 hastatus in there. They really do seem to be light on the load as far as water levels are concerned. I can't see the galaxies as being a heavy load either. If you can easily get hold of them maybe start with half a dozen and work your way up to suit.
My 10US gallon is overstocked since I had to move my 5 Aspidoras pauciradiatus in too. That gives me 6 pygmy corys and 5 pauciradiatus along with 2 Otos and 5 neons and 5 shrimp, lol. Heavily stocked ? (probably about 20 inches of fish when they grow up). In a week the tank's nitrates don't even register 10 ppm. I do a 50% water change once a week and the tank is planted which may help. I'm actually considering a few more neons to knock the school up to 8 so they'll be a bit happier.
I will say this tho, the Aspidoras and Corydoras all shoal together, they shoal around in one mass swarm which the pygmies never really did when I only had 6. They do split up and go food hunting sometimes but generally they all hang around a mob and intimidate the neons