I've had a N wennerae in a sump/refugium that's about 5 gallons for well over a year now. At his size a 2.5 would be fine if you can handle the pollution of feeding it. As long as you did waterchanges a 2.5 should be fine imo.
Room mates are two rock anemones, orange tree sponge, and zoanthids. Any other roommates have ended up as lunch...........