:I have a male crested gecko, about 7 inches, can i add another one which is only about 4 inches to his cage? :It looks like a female, but i guess you can't tell this soon. :Will they fight if it is a male? Is it too little?
I would not add the new gecko at this time. No matter if it is male or female, you only want to keep gecko's of about the same size together. This way they can defend themselves. I recomend that you keep them seperate for some time. Good luck with them David