Phew -- 6.1, man, that's going back now. And Sharding! talk about the archives of features 
I think the answer is that it's up to you to manage. A portlet is not deployed to a particular instance or to a site. The portlet exists at the server level-- so if you have deployed a TodoList portlet, then Site A on Instance A can add it to a page, but so can Site B n Instance B. It's considered a global asset for lack of a better term.
Same goes for the scheduled task. The task itself runs at the server level so if you need to do something for one company but not another, then it'll be up to you to look up the list of company ids, or the virtual host record from which you can get the company id, etc.
Sharding shouldn't matter at all as you simple use the services that Liferay provides and that magic is hidden from you.
I think the answer is that it's up to you to manage. A portlet is not deployed to a particular instance or to a site. The portlet exists at the server level-- so if you have deployed a TodoList portlet, then Site A on Instance A can add it to a page, but so can Site B n Instance B. It's considered a global asset for lack of a better term.
Same goes for the scheduled task. The task itself runs at the server level so if you need to do something for one company but not another, then it'll be up to you to look up the list of company ids, or the virtual host record from which you can get the company id, etc.
Sharding shouldn't matter at all as you simple use the services that Liferay provides and that magic is hidden from you.
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