My plan is to make the main site on the .net domain and als register the .net.pl and .org / .org.pl and maybe redirect those..
Thats not gonna help you, if anything you will be punished by google for that, nobody is going to randomly type in your domain with whatever domain extension, its a complete waste of money. The only thing that will achieve is stopping others registering your domain with a different extension and then out doing you on the SEO - highly unlikely.
I did that a few years ago, thinking it was a smart thing to do. I focused on the .com and the .co.uk became obsolete - totally useless except to stop somebody else registering it.
maybe not for serp but def for seo. ;)
Whats the difference? SEO produces the serp, its a combined effort.
with the same content, age and links a .pl will get u higher than a net.pl as it is top level opposed to second level.
domain extensions are not really the all of seo, in terms of whats really important its way down on the list providing you dont register a .eu or a .co.
From my experience and research theres hardly any difference in serp form a .com to a .net to a .co.uk or .com.pl, although my code is - if youre looking at a German market have something with a .de if youre looking at a UK market have something with a .uk beit .org.uk or .co.uk, for national ranking as in within a country the domain extension should be exclusive to that country.
.coms for lets say the search term 'property in Germany' generally rank lower than a .de within Germany for the same search term. But if your searching 'German property' in China from google.cn the .com may rank higher. If youre target marekt is Poland use a .pl extension.