Well they DID try and take over the world, starting with the tailed beasts then they began attacking konoha. If they won there they probably would have gone over to Suna then the Mist and other main villages. If they didnt stop the tailed beasts first they probably wouldnt be strong enough to just attack everyone all at once.
Plus their group isnt very orderly, some would probably run away if things became too difficult. I dont know whether they would all work together well in a group or if it would just be the pairs working together and a few people soloing which would make it easier to take them down.
I know they wanted to use the bijuus as a means of world domination but I just want to know why wouldn't it been easier to destroy the villages one by one. I'm sure Itachi wouldn't intervene once Konoha is last and it'll be too late then.
And you're right about them working in pairs to go to each village.
Suna village---Deidara/Sasori
Mist village--Tobi already soloed it before
Stone village-- Kakazu/Hidan/Preta path will be perfect to fight the tsuchikage's
Cloud village--Kisame/Itachi is enough because A, Bee, and Darui are the only threat
Leaf Village--Pain/Konan/Orochimaru----They'll be no problem with that.
Deidara was even successful in taking on Suna, after catching Gaara, if he dropped one more C3 or C4 the village would've been completely destroyed. Sasori didn't even participate either which shows their potential as a group.