I think they're already fighting somewhere else, like S-Snake and S-Hawk previously in Amazon Lily. Akainu made it clear the Marines are stretched right now, they need the Seraphim to fight for them.
"Let's put our faith in the SSG to do it's job." - Fujitora
We saw S-Snake and S-Hawk, a twosome, be deployed to Amazon Lily to take out Boa (this was actually really smart since Mihawk doesn't give a fuck about women - sorry Perona - and S-Snake would consider herself to be more beautiful than Boa due to her inheriting Boa's narcissism).
This is why Fujitora wanted the Seraphim over the Warlords, because people like Doflamingo and Crocodile gave nothing to the Marines while doing whatever they wanted, while the Seraphim can actually be used. The other two (or three) Seraphim are out fighting pirates/revolutions due to Sabo.
It's possible more Seraphim arrive at the end of this arc, too, then.