Are they going to get along or are they going to kill each other?
Well there are things you can do to help increase your chances of harmony between your new pets and old one. Having just recently added two new members to our crazy household I though I would share. I won't be focusing on the extra costs of owning extra pets today this will be about helping the pets get along. Also I won't be focusing on things like lizards since the likelihood of your leopard gecko interacting with your cat is slim to none provided you place his habitat in a secure location
First of all you need to consider what type of pets you already own. This not only includes species but breeds as well. For example... you don't want to mix a Rat Terrier with a Guinea Pig ...Terriers are born rodent killers and well Guinea Pigs are rodents so not the best mix.
Next if you have the chance rub an old towel on the soon to be new addition and leave it where your old friend can have a chance to sniff it. Animals are all about scents and this will help introduce you new friend to your old one before you even bring them home.
Next is real life introduction... having more then one adult helps immensely with this step a neutral location is suggested but I've have always just used my living room. In the case of introduction cats and dogs its always good to have an escape route for the cat ...if the cat wants to run away let them they can be very determined and will scratch and bite if scared enough. Now this next part can be a little hard on egos but you need to decide who the old pet prefers. No fighting now you have to be honest. One of the problems when adding a new pet is the old pet feels he's losing his place in the family. The pets favorite family member needs to sit with the old pet during introductions so he will feel reassured. Have another member sit with the new pet and let the pets meet. If you are worried about aggression the new pet can stay in their carrier while the intro sniffs are going on. If the pet outside the crate starts to act aggressive break it up with a quick sharp noise. Now obviously if you are introducing a small animal such as a guinea pig to an dog the guinea pig is the one in the cage. While they are getting to know each other its a good idea to keep them crated when you can't be there to watch them. It give the new member a safe secure place to be while you are not there.
Now the hard part... as with any change time plays a big role. You need to make sure that you are giving lots of love to your old buddy while he gets use to the new addition. Don't let the novelty of the new pet make your forget someone was there first. There will be some power play as the new pets figure out the dynamics of there place in the house and as long as it doesn't get out of hand you need to let it happen. If it does start to get out of hand NEVER stick your hand in to try and break it up. Try a spray of water and if that doesn't work try a more physical barrier such as a couch cushion. Once separated then remove one of them from the room and let things settle down. Once they have determined their place in the home peace should return.
* sometimes there are instances where pets refuse to get along in these instances you have to decide what is best for your pets.
** alot of this is kind of geared toward introducing dogs to dogs, cats to cats, or dog to cats. for example you wouldn't leave a hamster or pet rat running around the house.