#clearrec doesn't affect units' abilities, it only applies to units a nation has access to (can recruit from every castle). I think you confused it with #clearspec, which would take all special abilities away from a creature...
Also, if you hit some kind of a maximum value, I don't think it was from the nation. When I made the Unit Replacement/Null Mod series, the different nations could replace each other almost any way. If Ulm can replace Abysia and Abysia Ulm without there being any chances I could notice, there should be no problems with adding units.
You can't use different nations' sites, but you can match sites from the themes. If you modded e.g. Warlocks of Smouldercone, you would change Abysia's capital-only units too. Also, I think only Abysia (or modded nation replacing it) would be able to recruit the units from Smouldercone, regardless of who starts with it. However, there are few annoying restrictions to using themes' sites. Man has two other sites in its Last of the Tuatha theme: one is named "Forest of Avalon" (the same as the base theme one), the other "Avalon". Only Avalon can be added as a site, because Forest of Avalon finds the first FoA, the one for the base theme, and there is no way to make a difference between them.
Also, you can't actually have the Zultu appear as a theme. You can use the Golden Era's sites as you can use any other site in the game, but you can't mod a theme with the nation modding commands.
You could replace all the units/commanders with modded ones, but it's not worth it, as modding normal nations using the theme's sites gives you much finer control over the result. However, I think Golden Era would still show up as a theme under the chosen settings... It'd be an interesting theme if themes were moddable, though, because it has forced Sloth scale.
I quess using the Knight heroes as a template for Sorcerers would be all right. The sorcerers needn't be much better than your normal ones, though. The chance to get them isn't very big even with Luck 3, but 13 Angerbodas or even Blind Ones would be a bit much. And remember that some people use forging Clam of Pearls, massing up Astral income of hundreds and chain-casting Wish as a strategy on the big maps... Some people do recruit more than 13 high-tier mages.
