Guild of mercenaries.

So, if your clan suddenly seeks help from mercenaries through the guild, it won't get to choose its candidate, but will have to agree with any candidate the guild provides, is that right?

First of all, as I already said, we will not be seeking mercenaries here.
Secondly, even if we do seek "help" here (which is unlikely), we will not take just any mercenary that comes along. And I think that mercenaries will be tested for their skill in the game directly by the clan's players.
I believe that if the Guild exists on HeroesWorld, then it should consist of experienced players.

and why can't beginners join right away?

They can, but who will pay them any attention?