5 Elder Stars are aliens, disguised as humans, or they could be using Ope Ope no mi's Eternal Youth ability (Oda mentioned this for a reason), and Doflamingo was aiming for becoming one of the Elder Stars by having the Eternal Youth via Ope Ope no mi.
Imu could be an alien or using Eternal Youth ability as well, just like Gorousei.
Oda has many options with these guys, and there is a reason he made Admirals weaker than Yonko, and top Yonko underlings like Marco, Beckman, Shiryu, Oden, Katakuri etc. won't lose to the Admirals for this reason. The real threat should be the Gorousei, Imu and Vegapunk's weapons.
Or they could be just 5 Spandams or old retired fighters from CP0, it could be anything.
It's also possible that the marines can split up to 2, or they can be divided into 3 groups, like Group1: SWORD, Group2: Fujitora / Greenbull, Group3: Akainu / Kizaru. I doubt Oda will let Akainu to lead the entire marines for so long. Akainu will fail, he won't be successful and he won't be an effective leader like Sengoku.