The smallest cage I recommend for a pair of finches is 30X18X18. That’s because a flying bird is a healthy bird. You want them to fly around.
Did you notice that the perches that came with your cage were all 1/2-inch dowels, and they’re extremely smooth? Yeah. That’s not what we want.
Gouldians need to be able to fly in their cage; it’s good exercise and it keeps them healthy. Hopping from perch to perch doesn’t cut it. So before you do anything else, please buy at least a 30X18X18 cage for your finch.