How can my 3 cats and 3 Chihuahuas get along?

We have 3 older female cats ( 8-12 yrs old) and then got a male chihuahua 2 yrs ago, a female chihuahua 1 yr ago, then another male chihuahua puppy 6 months ago. The cats used to hang around downstairs until the female chi started chasing them and the male chi's would join in. The youngest cat still hangs out downstairs however she stays up high. The other cats stay upstairs and one of them has pooped upstairs probably because they could not come down and use their box. The cats are inside, the dogs get walked nightly 1 1/2 miles and frequently run in the backyard. The female chi seems to be the aggressive one. Is there anyway to restore harmony?