I think if it were my tank I'd refrain from adding any fish that is at all in the 'fragile' category (anything that moves slow or has long fins or is stressed out/scared easily). TB's and the RTBS are pretty tough guys...they go well with fish like Danios that can take care of themselves
I'd be careful of adding anything in the lower level of the tank like loaches because the RTBS gets more territorial and more grumpy as it gets bigger and older.
The tank might be long enough to get a pleco or a group of corydoras, either of those would probably be fine even as the RTBS gets older, as long as each of them have their own hiding places, caves, driftwood something.
SO lets see...with those inches...I'd probably get a few more tiger barbs (same size as the ones you have) and a group of about 6-8 zebra danios (not the long finned kind) and either a pleco or some corydoras, something that adds up to another 6-10 inches or so would be good. Oh and an apple snail because every tank needs one
Just remember no matter what you get you need to add them a few at a time so your bacteria can catch up to the extra load.
If your tank isn't cycled (or you dont know what that means) then I dont think you should get anymore fish right now...but you should get some test kits if you dont already have some and read around the boards a little more and wait for your tank to cycle