How to tell if a baby leopard gecko is male or female to avoid fighting?

Unfortunately, it is common for intact males of the same species to fight, so you may experience that as they get older. They can be very territorial, and it doesn't matter if they are siblings or not. As you mentioned, male and female geckos can (and often will) mate, no matter if they are siblings or not. If you have two females, this would be the most ideal housing situation for multiple animals. Females will often tolerate one another in the same area, however there are exceptions to the rule, and they can fight as well.

I commend you for rescuing those Leos! Female leopard geckos may live together peacefully as long as they have a large enough enclosure. Males should not be housed together as they will fight. If you have a male and female, they should be separated until the female is at least 55 grams, which is the minimum weight recommended for breeding. I would guess they may be about 3-5 months old. Hatchlings typically weigh 2-5 grams for reference. There is no "standard" since leos will grow at different rates depending on dietary and environmental factors. You can determine sex by looking underneath. Males have noticeably distinct femoral pores and pre-anal pores. The base of their tail will have two hemipenal bumps as well. You can review leopard gecko care here: http://www.anapsid.org/leopardgek.html

Keeping them together would not be recommended. The adult may see the juvenile as prey and may try to eat him/her. Cresties can be tricky to sex, especially when young. If yours weighs at least 10-15 grams, you may be able to see pores using a magnifying glass or jeweler's loupe. You may see the beginnings of a bulge at around 30 grams body weight. If you are unsure, have him/her examined by your veterinarian or a vet experienced in treating reptiles.

Leopard geckos can generally co-exist with one another as long as they are male-female pairs or two/more females. They usually do not become territorial while they are young. That being said it is important to make sure they have enough space to escape each other and have more than one hiding area. .If you see bullying, you may need to separate them.

Two female geckos cam live together peacefully as long as the enclosure is of sufficient size. Two males should never be housed together. Gender cannot be determined by the images posted. Males can be differentiated from females by the presence of a V shaped line of pre anal pores just in front of the vent and hemipenal bulges behind it. Your gecko appears to be in the middle of a shed. This is a stressful time for geckos. Provide one or two moist hides in the cage and daily soaks to aid in shedding. Feed appropriately sized prey, no bigger than the space between the eyes, that have been gut loaded for 24 hours prior to offering to maximize nutritional value. Calcium w/D3 and Vitamin supplementation is essential to good health. Consider an exam with your veterinarian or an experienced herp vet to diagnose illness or injury. Submit a stool sample to diagnose intestinal parasites.

Males are difficult because they can be territorial. This can result in aggression and potential injuries. Mixing different species is never recommended. Introducing a female may be your only option, but you risk breeding and unwanted or accidental clutches of eggs.

Juveniles smaller than about 4 inches are difficult to sex. Once they reach that size or larger, it is quite simple.Female sliders are larger, have shorter front claws and thinner. smaller tails than males. Males can be territorial and could "argue" if the tank is not of sufficient size. Ideally, you want to allow about 10 gallons per inch of turtle. Sliders can get big (12 inches fully grown), so be sure you have the space and budget for a large tank.