Where did vegapunk or anyone said seraphim differentiate in strength, he called them strongest creatures, didnt say one stronger then the other, snake=bear=hawk=shark.
Saying one is stronger then the other just because the warlords are is headcannon
Neither has anyone said that this warlord is stronger than that warlord. All of them shared the same rank but first we have some fights between them and see that this is clearly not the case and second we have eyes for ourselves to judge some things.
It's not headcannon to say mihawk > boa based on the things that we have read.
Now for the Seraphims they have warlord's appearance, skills, power and personality that are having also lunarian DNA, giant DNA, px laser and a df ability. The base of a seraphim is the warlord because of how they fight or even behave. Vp added powers to a warlord.
If we take Luffy and ussop for example and create two Seraphims, of course Luffy's will be the strongest one.
I wonder if this headcannon thought of mine, is still headcannon if two strawhats beat a seraphim and those strawhats are far apart in pl like sanji and Franky for example