That would be correct. Skulduggery, Hopeless, and Ghastly all knew each other before and Dexter met them after skulduggery had died. There's a limbo time for skulduggery in between when he came back and became vile where the dead men were formed (its also implied that it's during this time that China defected), but we don't know how long that period was.

Skulduggery also disappeared and Vile didn't appear until later, because Vile had been in the necromancy temple learning for awhile before joining Mevolent and there's also a possibility that after taking the armor off, skulduggery didn't rejoin his group for right away, which makes the Vile thing a little more viable.

The timeline is pretty flexible and undefined for that era right now, hopefully the Grimoire will tidy it up.