They can grow up to almost a foot (total length) and be robust and tall in the body with very pretty extensions on their fins. They can live for 30 years (!!) if well cared for and no accident claims their lives - so having one of these fish is a 2-3 times longer commitment than having a cat or a dog! People do not think about that.
http://www.planetcatfish.com/forum/view ... =8&t=30928
They can be territorial with conspecifics as well as other synos but this depends on the tank size and the set up/hiding places/furniture. They will become much more visible with other fish and other synos around. The reason you don't see him is you got so many hiding places there and no dither fish or at least just other fish (other than the two bettas). They live in bare tanks too - this is a bit stressful for them but dim light will alleviate most of that stress - after all, this is how they live at LFSs sometimes for months, 0.5, 1 year before sold.
Another reason may be that bettas attack him. So he hides. Even food would not lure out a fish that is being beat up.
I have had up to 6 of them at times in large, diverse community tanks. They always come out when food is dropped in and feed enthusiastically. They spend all night looking for crumbs too. They are riverine synos, not lake synos. Riverine are always more reclusive and hide more.
They are one of the most common synos that are out there. People do not realize how big they grow and that they are semi-aggressive. Hence, you often see 5"-10" trade-ins at the LFSs for $25-$35. Young fish is never more than a few bucks. Maybe $5.
Welcome and hope this helps.