I have limited experience with Denison's, so somebody else may have different advice. But, from what I have personally seen, I would doubt that they will generally swim much higher than the lemon tetras do. If you want some more filler in the upper regions, I would consider hatchets (make sure to have a good cover) or perhaps harlequin rasboras which seem to stay higher in the tank than many other schoolers.
Honestly though, if you really want Denison barbs, I would say go for it. They are wonderful fish and I really doubt that their movement would spook the geos. You can always trade them out for something else later if you decide the combo isn't quite right. In my opinion, there is nothing wrong with a bit of trial and error with getting a good mix as long as you can keep the parameters in check and you are not dealing with very aggressive fish that might damage each other. Also, so long as you are able to responsibly rehome the fish you need to remove.
Two roads diverged in a yellow wood, but thankfully they both led to fish stores.