Having seen all the rules about adjective declension after numbers in the nominative case; I wonder if I just can use the genitive plural in all this kind of adjectives without hindering a conversation with a native. Thanks.
This will certainly be understandable. But I'm not sure Duolingo will accept that.
Normally when you mean 'kitchen' you say на кухне not в кухне, likewise на кухню, с кухни. But в кухне, в кухню, из кухни are also possible. You can encounter both in contemporary conversational language and in literature.
If you mean 'cuisine,' however, you use в.
My understanding from my Russian tutor (who is from St Petersburg but has lived in Moscow for years) is that in Moscow and increasingly in St Pete, people say на кухне to mean "in the kitchen." Elsewhere in Russia, в кухне is the common way to say it. So which phrase you use will carry some cultural baggage, but will be understood either way.