well for the sump since its already predrilled with an installed overflow, just let the water overflow into the 20g and plumb a pump from the 20g into the 65 and that's pretty much it. Just look for schematics to make the 20g into a sump where you can put all the filter media. you won't need an ac110 with a sump configured correctly. You can go with some medium light stem plants that grow slightly slower such as ludwigia repens. But i would suggest having some fast growing stems until your tank stabilizes that's easy to remove such as hornwort.
FYI most vals HATE excel and other liquid co2s