A) (Regional)
Arbok, Beedrill, Lapras, Porygon and Rhydon (Gen 1),
Ledian, Noctowl, Scizor, Stantler and Xatu(Gen 2)
Armaldo, Flygon, Milotic, Relicanth and Torkoal (Gen 3)
Bronzong, Floatzel, Porygon Z, Spiritomb and Vespiquen (Gen 4),
Garbador, Gigalith, Seismitoad, Stunfisk and Swanna (Gen 5)
Aegislash, Heliolisk, Klefki, Malamar and Tyrantrum (Gen 6)
Dhelmise, Drampa, Mudsdale, Tsareena and Vikavolt (Gen 7 )
Appletun, Coalossal, Dragapult, Sandaconda and Thievul (Gen 8 )
Baxcalibur, Brambleghast, Garganacl, Houndstone and Tinkaton(Gen 9)

I will consider Porygon-Z voided as you already mentioned Porygon for Generation I, so you can select another Gen IV Pokémon (that does not already have a member of its line mentioned) if you would like.
That aside, Nox Venem's regionals are looking like quite a selection, lol.

B) (Convergent)
Clefable, Golem, Kadabra, Parasect and Rhydon (Gen 1)
Aradios, Forretress, Shuckle, Smeargle and Xatu(Gen 2)
Beautifly, Ludicolo, Masequerain, Nincada and Swellow(Gen 3)
Hippowdon, Kricketune, Luxray, Roserade and Spiritomb (Gen 4)
Carracosta, Crustle, Ferrothorn, Klingklang and Sigilyph (Gen 5)
Avalugg, Barbaracle, Goodra, Heliolisk and Noivern (Gen 6)
Drampa, Kommo-o, Palossand, Toxapex and Turtonator (Gen 7)
Corviknight, Dragapult, Drednaw, Sirfetch'd and Thievul (Gen 8 )
Cyclizar, Garganacl, Gimmighoul, Scovillain and Tinkaton(Gen 9)

A-OK here!
Nox Venem's current convergents will begin from this page.

C (Third Stage Evolution)
Fearow, Kabutops, Kingler, Pinsir and Tentacruel (Gen 1)
Donphan, Heracross, Houndoom, Porygon2 and Scizor (Gen 2)
Camerupt, Claydol, Sableeye, Sharpedo and Wailord(Gen 3)
Carnivine, Drapion, Gastrodon, Gliscor and Yanmega(Gen 4)
Beheeyeem, Cryogonal, Druddigon, Heatmor and Stunfisk(Gen 5)
Carbink, Clawitzer, Dragalge, Pangoro and Pyroar (Gen 6)
Dhelmise, Gumshoos, Mimikyu, Minior and Turtonator(Gen 7)
Barraskewda, Copperajah, Pincurchin, Sandaconda(Gen 8 )
Dodonzo, Dudunsparce, Houndstone(Gen 9)

Alright, you only listed four for Generation 8 and three for Generation 9, so those left out can probably be thought of at another time.
(Kleavor is treated as part of Scizor's line, so that one does not count.)
Nox Venem evolutions beginning here.

D Dark, Dragon, Ghost, Electric

Then those are the types I will use for the Xeniu-Midon Elite Four.