How many Underworld's emperors were..? Why Big Mom invited them to Tea Party..?

Underworld..
We 1st knew about this 'Underworld' at Auction House in Shabaody Archipelago..
The place where Camy been kidnap to be sold at high price..
The same place we met Silver Rayleigh (Gol D Roger's crew member)..
That Auction House belong to Joker (Doflamingo the warlord)..


Then, Oda told us about Germa 66 who got link with Underworld..
Are Vinsmoke also one of the Underworld's emperor..?
Are 5 head in West Blue included Capone 'Gang' Bege also the Underworld's emperors..?
Could it be Joker (Doflamingo) also one of the Underworld's emperor..?
Or there are others (beside the one i mentioned above) are the Underworld's emperor that will attend Big Mom's Tea Party..

What about The Revolutionery Army under Dragon..?
Is Dragon one of the Underworld's emperor..?
Or there will be a member under The Revolutionery Army that is also one of the Underworld's emperor..?

Is Crocodile one of the Underworld's emperor too..?
Could it be that every Mafia/Gangster/Yakuza could be the theme for Underworld's emperor around the world..?

Then, why Big Mom's invited all Underworld's emperor to her Tea Party..?
To witnessed the downfall of Vinsmoke..?
To threaten Capone 'Gang' Bege..?
Because Big Mom was one of the emperor in Underworld..?
Or because Big Mom had one of the Underworld's emperor as her husband..?
We seen Big Mom's little daughter that always hold knife as she want to destroy her stuffed animal.. Could it be Big Mom married one of the Underworld's emperor..?
Remember also that Big Mom' crew members wear black glasses (maybe in term of assassin attire - one of style that every mafia/gangster wears)

What do you guys think on this topic..?
Share your thought here..
Please COMMENT.. ^^