Don't misunderstand me.
When you get a job of course! You must sign a contract, but you can have more than one agent and don't need to agree with all what the agencies ask.
If a TV commercial will be done within a week, you don't need to sign 1 years exclusive contract;
you sign a contract for that job and keep the opportunity to work with other agencies.
An exclusivity clause forbids you to work with other firms, thus when other agencies offer you higher payment or better jobs, the first agency will take advantage of the exclusive agreement. (RED FLAG) Good deal for them but what is the benefit for you?
When a company asks me to sign an exclusive contract, I request (at least) guaranty incomes.
Notice that it is not only for overseas models.
Regarding the commission, I imply 40% is the average but I know some companies charge higher rates.
Some agencies don't even work by commission, instead they pay low wage to the actors/models and take most part of the incomes.
I am trying to make things more transparent so new actors don't be tricked so easily and actors with some experience (like you) can improve their situation.