Migration assistant to move to a Mac that's not new?

I have been using two Macs for a while for various reasons, and now I want to consolidate my stuff onto one of them. I want to move Downloads, Documents, Desktop, and Pics/Videos. I have questions about using Migration Assistant for this.

1. Can I even use Migration Assistant to move files onto a Mac I'm already using (i.e. without wiping it)?
2. Does it make sense to use Migration Assistant, or would it make more sense to simply copy stuff over in a more manual way (e.g. using an external hard drive as a middle step)?
3. If the admin username / password are identical between the two Macs, will that cause any issues?