How to take that into account?

I see no constaint about this strucutre. Could you point out more precisely your issues ?

For instance is there a solution so that managers are aware of real work allocation entered for that person in the other team support activities? I see no way to give one manager a read-only access to other team support project.

This possible to through access controls.
Just define a new access mode (or change existing one) that will give "read access to all projects", and "create/update/delete access to own projects".
Then define this access mode to "acces mode to screen" for project leader profile.

Should we use a common project for holidays?

Yes, and it should be an administrative projet (this type is designed for this purpose)

My problem is that we don't want to see all the project of the other manager but only the ones in which the shared people is assigned (i'm one of the manager), for privacy reasons and because we have a lot of small projects.

Until now, Projectoria was only used for my own team, i was the admin and the only "project leader" and life was easy.

So my knowledge with multiple teams is very low. Several question, if you have time:

- does the "manager" field of a project changes access rights or this this just an informative field? I guess no (as i have the surprise to have no more access to some project for which is was the manager, when i switch to admin account from my own one with "project manager" profile)

- same question for the "issuer" and the "responsible" field of activities

- if several ressources with "project manager" profile as assigned to a project, they all have write access to that project?

For holidays, can you briefly explain the advantages of an administrative project over our "solution" :

- We need to track remaining holidays, so we create an "internal" project for year 2013 with all the people of my team and number of holidays as assigned work.

- We then "fix planning" of the project so that no automatic planning is done (but we also plan to use "regular between dates" planning type to preallocate holidays)

Thanks

- does the "manager" field of a project changes access rights or this this just an informative field? I guess no (as i have the surprise to have no more access to some project for which is was the manager, when i switch to admin account from my own one with "project manager" profile)

Manager is only informative field. It is only used to be able to send mail to "Project Manager".

- same question for the "issuer" and the "responsible" field of activities

Issuer is only information (same use as Manager").
Responsible has some automations depending on configuration : possibility to automatically create assignment for responsible.

- if several ressources with "project manager" profile as assigned to a project, they all have write access to that project?

Yes.

For holidays, can you briefly explain the advantages of an administrative project over our "solution" :
- We need to track remaining holidays, so we create an "internal" project for year 2013 with all the people of my team and number of holidays as assigned work.
- We then "fix planning" of the project so that no automatic planning is done (but we also plan to use "regular between dates" planning type to preallocate holidays)

Your wait to work is good.
Administrative project would bring additional features :
- everybody can enter real work on administrative projet activities, without having to be affected nor assigned (and assigned / left work can be adjected afterwards)
- administrative projects are never planned (without having to check "fix planning")
- administrative projects dont appear in the gantt view.

Thanks for all you explanations