Hey, I have an AWESOME ONE! It's called Gravity 3D, its hard to figure out, so read the ReadMe first.
You can have up to 12 galaxies interacting... sadly... you can't place them. You can try to make files but it doesn't tell you what the values mean. I wish you could have a cluster of 10 and 2 on the outskirts. Dan, maybe you could figure it out?