Always ensure you introduce a gecko of a similar size and age, this will minimise the likelihood of fighting. Size difference can encourage dominance of the larger gecko...but then it depends on how much smaller this gecko is... Keep a good firm eye on them for the first few hours of being together - they may seem ok at first but they can change their mind in a heartbeat. Also be aware of any changes in appetites or behaviour which may indicate either gecko is uncomfortable or scared (tail waving, pressing their bodies to the ground and keeping very low). Also provide new hiding spots which the already resident gecko hasn't used before to avoid territorialism - made sure there are plenty of places for both geckos to retreat to.