The hired squads function as a lopsided regular slayer (real person) favouring attack or defence, but in addition give you a 2% boost to either attack or defence respectively, up to the preferable 10% hired boost (5 x 2% = 10% )
Defenders
When combined with squad contracts it gives up to 15% defence boost, which rather than adding actual numbers i'm pretty sure it's just there to cancel out any portion of the opposing offensive boost of the person attacking you.
Strikers
Same again, when combined with the offensive squad contract it gives up to 15% offensive boost, only this time can have the 5% vaccine boost stacked on top of it for a maximum of 20% boost to attacking power.
For this reason, I believe attackers almost always have the advantage as 20% is better than 15%. Say the 15s cancel eachother out if you understand that.. Ignore the fact both players could have 15% boost from 5 hired squad and the respective squad contract.
This leaves just the possible 5% boost from the temporary vaccine most commonly picked up before visiting the fight or rivals list (or possibly even the hitlist, but i don't recommend that in most cases because you kill even if you lose, and what if no bounties appear in the time the vaccine lasts?
Of course there are stamina boosts which can be stacked on top of the Striking boost, but i wouldn't recommend that as, well to be honest, you will be mocked / laughed at / probably killed and/or hitlisted for your cheekiness. ESPECIALLY IF YOU STILL LOSE. LOLLL
These should only be used, well... I don't know. Like i said, when you want to be mocked, laughed at and punished by the person you use it on alongside the people sharing the chat group they are using.
Alongside that fact it dramatically reduces the xp per stam payout of the fight (does it decrease the xp given for a win if a stam boost is used? I know a failed stam boost seems to give the receiver a nice tasty experience treat)
Uhmm did i go too far? Hopefully you can get your head around the boosts provided by the hired squad and how they can be combined with the other boosts on offer. If not, just try to explain your confusion and I (or someone else) can try to explain it more clearly, or more specifially...