Yeah that interaction works out. If both Zegana, Utopian Speaker and Sage of Hours are in any graveyards and you have a Necrotic Ooze and some way to boost +1/+1 counter creation, you will get to take extra turns for 4UG.
Sultai are great color for a graveyard commander deck, Sidisi, Brood Tyrant, Muldrotha, the Gravetide and The Mimeoplasm all make awesome graveyard deck commanders. Including Zegana, Sage and boosts to +1/+1 counters suggest a heavy thematic focus on +1/+1 counters, so if you want to be doing that, maybe Atraxa, Praetors' Voice would be the go-to commander. If you go all-out on the +1/+1 cou nter theme, there are lots of cards and very nice combos and synergies. If you rather want to focus on your graveyard theme, I would recommend to go for overall synergies there instead of specific combo pieces that have little functionality on their own. E.g. Necrotic Ooze works super well with other graveyard goodies such as Havengul Lich, Whisper, Blood Liturgist and Avatar of Woe...
You will probably get better results when focussing on one theme OR the other, but commander is a format all about doing whats fun for you, so if you want that +1/+1 counter graveyard combo deck - you do you!
Sultai are great color for a graveyard commander deck, Sidisi, Brood Tyrant, Muldrotha, the Gravetide and The Mimeoplasm all make awesome graveyard deck commanders. Including Zegana, Sage and boosts to +1/+1 counters suggest a heavy thematic focus on +1/+1 counters, so if you want to be doing that, maybe Atraxa, Praetors' Voice would be the go-to commander. If you go all-out on the +1/+1 cou nter theme, there are lots of cards and very nice combos and synergies. If you rather want to focus on your graveyard theme, I would recommend to go for overall synergies there instead of specific combo pieces that have little functionality on their own. E.g. Necrotic Ooze works super well with other graveyard goodies such as Havengul Lich, Whisper, Blood Liturgist and Avatar of Woe...
You will probably get better results when focussing on one theme OR the other, but commander is a format all about doing whats fun for you, so if you want that +1/+1 counter graveyard combo deck - you do you!
UR Mizzix of the Izmagnus ~~~ Build your own win-condition: Finite Spellslinging
UR Brudiclad, Telchor Engineer ~~~ We are the Borg. We will add your biological and technological distinctiveness to our own.
WUB Oloro, Ageless Ascetic ~~~ A Guide to dying slowly
UBR Marchesa, the Black Rose ~~~ Marchesa's undying Marionettes
RGW Mayael the Anima ~~~ All Hail the Big Chungus
GWU Chulane, Teller of Tales ~~~ Permanents Only ETB Shenanigans
BGU Sidisi, Brood Tyrant ~~~ Sidisi's Restless Servants
WUBRG The Ur-Dragon ~~~ Dragons eat your face