The tincs can be aggressive toward each other. I doubt they would attack the gecko. The gecko attacking them, well, I am not so sure. I have never kept Phelsuma klemmeri, but I have kept P. madagascariensis, P. lineata, and P. quadriocellata. Sometimes they can be a little aggressive, at least the madagascariensis can be. I know P. klemmeri can be one the smaller species, but I have never seen one in the flesh, so I really have no concept of how truly small they are. They may work out, but I can't say for sure.
The thing that really have to consider is the origin of both species. Neither can be wild caught for this to work. You don't want either to infected the other with exotic pathogens, and wild caught darts are definitely toxic. So mull it over and consider the risks before advancing.