Are the cats all fixed? Expect more trouble with the females, it's the queen cat who runs the house usually. Don't be surprised if one of them takes to not using the litterbox, that happens often. Try leaving the new cats in carriers in a room with the others to get them used to each other. I have more luck bringing in kittens than mixing older cats and once had to keep an adopted older female in a laundry room for 3 months till I found her another home because her and my established queen went after each other for the kill everytime they saw each other. Just give them time and have patience and hopefully they'll learn to get along. Make sure you pay them equal attention. I'll try and think of some other tips.