Learn Croatian: Znaš li nekoga iz Engleske? - Do you know anyone from England?

You are right that in the genitive case, both regular feminine nouns (like žena -> žene) and feminine adjectives end in -e. However, Engleska takes adjective endings in all cases. You really notice the difference in the dative and locative cases: 'in England' is u Engleskoj (taking the feminine adjective ending -oj), rather than taking a regular noun ending.

Shouldn't we use poznaješ instead of znaš when talking about knowing people?

Technically, yes. Traditional grammar teaches that znati is for facts and skills, while poznavati is for being acquainted with people and places. However, in everyday spoken Croatian, it is extremely common to hear znati used for people as well, especially in casual questions like Znaš li nekoga...?.

Why is the pronoun nekoga used here instead of netko?

Netko (someone/anyone) is the nominative form. In this sentence, the person you might know is the direct object of the verb znaš, so it must be in the accusative case. The accusative form of netko is nekoga.

Could I use the preposition od instead of iz here, since both can mean 'from'?