Very interesting project!
Might I suggest looking at the Warlord (former Wargames Factory) and/or Wargames Atlantic skeletons too? those are not sculpted heroically, so would probably go together with Warlord legionaries much better. Also, splicing the smaller skellie heads into spare roman helmets will most likely yield much better results.
The Mantic undead are great too btw, but more limited in their conversion options. The ghoul and zombie 'ranges' each only contain 2 miniatures, and the skellies, whilst awesome, are difficult to convert as they're all done up in 15th century(ish) armour.
And aren't the Svezda miniatures more 1/72 scale?
I'm looking forward to see what you come up with though!