Personally, I think the "generic" post-marian legionnaires should still be available when the first round of named and numbered legions become available (so, Roman Fortress?), but then disappear completely when the Curia Hostilia (representing Augustus' reforms) is built, as at this point, you haven't got the 28 legions yet. Most of the local units stay available between the two buildings, so there's no change there, just that the Roman Fortress should add the early named and numbered legions to the roster available under the Campus Martius, and then when you build the Curia Hostilia, there's the sweeping change of Augustus' reforms that removes everything but the named and numbered legions (or at least, moves them around), and introduces the segmentata legions, and replaces the local units with regularised Auxilia.
What do you think, DVK?
This would simulate the Roman army of the very late republic - from the time of Caesar and Pompey to after the Battle of Actium - as 60 legions (including some legions which were named or would go on to be named, but most were just numbered as they were recruited) were extant at the end of the Republic, and Augustus disbanded and merged them down to a much more manageable 28 named and numbered legions.
Also, the Evocati are retired veterans recalled to the standards - they're good, but not the best combat troops available, because, well, they weren't! They're good for garrisoning your cities, and they make passable second line troops, but they should never be the mainstay of your armies, because they weren't ever the mainstay of an army.
