If a unit is corrupted, can it still be assigned damage? I think yes because being corrupted just means that you can not attack or defend, that doesnt mean that I cant attack the unit and it would still gets its toughness defence and things like that. let me know what you think and thx in advance!

You can not attack units directly unless you have a card that allows you to do so. Like lure them out. But corrupted cards can still be assigned damage if they're corrupted after they have been declared as defenders.

Is it allowed to assign damage from a former uncorrupted unit who becomes corrupted after it was declared as an attacker (like Clan Rats)?

Yes. It's still considered an attacker and will take and deal damage normally.

before battle you declare whitch zone you want attack, you can't declare whitch unit you gonna attack - opponent player do it. So if unit is corrupted defending player can't choose corrupted unit as a defender becouse as it is corrupted it cant defend. So if it's only unit in zone under attack, damage goes directly to the zone. However you can assign damage to corrupted unit by special effect from tactic card, or action. That's how I understand this rule