6 terribs in a 40b vert (16x18 footprint IIRC?) even without the geckos seems like a waste of money to me, since that's hardly enough squatting space for two and you're asking for trouble.. and if your geckos ever breed you can pretty much say goodbye to the babies with terribs roaming around. I won't flame you but I just flat out don't see it turning out well.
For your other 40b vert, look into variabilis, lamasi, or vents, they tend to do better in groups. They are a little more shy than imis/most pums, but once they're old enough to breed they'll be focused on getting some action and you'll see them plenty. I didn't see my Iquitos reds or variabilis at all for the first 6-8 months, but now that the breeding behavior has started they're out constantly and aren't skittish.