One thing's for sure: The Ravager & Leviathan are not the same design. The Leviathan has a rounded shape near the front, and a defined gap. The Ravager was more wedge shaped and streamlined (probably not as big either). They aren't exactly the same as ISD's, which have a more defined bridge tower.
I believe they were both Republic ships, designed with a similar architecture model (which was used even during the Clone Wars 4000 years later). It was suggested the Star Forge could replicate models, thus the Leviathan design could've been used as a template by Revan & Malak. They never explicitly said the Leviathan were alien design either.
The Endar spire & Harbinger style ships were much different, probably older, type of warship. Most of the Leviathan/Ravager ships, if not taken by Revan, were destroyed in the Mandalorian Wars, leaving little to the Republic fleet.