Resident cats fighting with each other with introduction of new kittens

Hi,

about a week ago, I took in two four month old feral kittens I had been feeding. I also have two adult cats -- ages 14 and 5 years old, who have always gotten along fine. 

Of course they hissed at the kittens (who I have been keeping in a kitty play pen - - I live in a one bedroom apartment) - - but over the past few days they stopped and seemed to be tolerating  - - if not accepting the kittens fairly well.  I've been doing my best to give all four cats equal attention. But tonight both my resident cats had a scuffle. I didn't see how it started but I heard a commotion in the other room and then they both ran into my room and starting hissing, swatting, etc. until I yelled and they stopped. So far - - no other incidents. 

I was surprised - - although when I think about it  - - I probably shouldn't be. Their territory has been altered and now they have to battle it out for status. But I am concerned about leaving them alone together and it makes me wonder if I should keep them separated when I'm not at home. This may have been the first I've seen of it but that doesn't mean it hasn't happened before.

Any advice?

Many thanks in advance!

Like you said the presence of the new kittens altered their ways. It happened to me long time ago. For safety i kept them separated when i wasn't home, and while i slept. Had to reintroduce them, and after that they were buddies again. Also, a vet check up won't hurt. Sometimes when cats get sick their scent may change, and the other will "see" his companion as a stranger. I'm sure the cats are ok, and all is because of the new little ones, but is better to make sure since you have a cat who is an older teen.

Definitely keep them separated while you're not home. 

It takes a long time for resident cats to fully accept newcomers, unfortunately. There are of course exceptions but most cases it does take a long time, especially if they are older cats since they are kinda "stuck" in their own ways. 

Have you looked into scent exchanging or mixing scents? Take a worn gym shirt filled with your scent, rub it on the resident cats, and then rubbing it on the kittens. The key is to let the cats know that although there are new scents in the house, their old scent is still there. 

Also bringing all the cats to the vet (if you haven't) is a good idea to rule out any health issues. Have you also tried a Feliway diffuser? Play some relaxing harp music? How many litter boxes do you have? You may need to increase the amount of boxes so there's less territory issues.

Although it's difficult do give the older, resident cats more attention. Don't force it but leave them treats, or just sit with them for a bit, talking to them. 

Thanks! They're both pretty laid back friendly cats so I was surprised when they acted out like that. But then - - they are cats

Of course my 10 year old basset hound is completely oblivious to it all and slept right through the fight.  I was up all night last night listening for more. So I  will definitely keep them separated from now on and when I leave. 

Thanks Pinkman! Oddly enough I have done scent mixing - - but when it was when I introduced my new (at the time) basset hound to the older cat. But - - I didn't have to do it when I introduced my younger cat back in 2009. She fit right in. No fights or hissing (well at the dog a few times). Which is why I was so surprised when they fought last night.

I will definitely try it now. They are both food motivated so that should work. Also Mischief - -  14 year old cat is being treated with Prednsolone for a skin allergy. The younger cat - - Orson, hasn't been to the vet for awhile. So she is due for a checkup

Update: I called the vet - - she said since Prednsolone is a steroid, it can cause aggression in some cats. So that combined with the new kittens being introduced may have caused the outburst. I'll still keep him separated until the medicine has run its course. Thanks again to everyone for their responses!