Generally, it's preferable to not use subject pronouns. This wouldn't work in English because we know who is doing what based on the subject pronoun, not the verb. I read You read We read They read But in Spanish, and many other languages, you can get the same info through the verb alone: Leo Lees Lee Leemos Leéis Leen So, generally you don't need the subject pronoun. You can add it for clarification (él lee, ella lee, usted lee) or for emphasis (él no lee, *yo* leo). But if you add it a lot when you don't need to, particularly with "yo" it sounds like you're drawing attention to yourself. *I* speak a little Spanish.

I'm not sure, but i think the full form is "a little of" that's why we add "de" in the sentence, but also, it is for phonetic purposes. If you want to write the sentence without "de" then remove "un" so the sentence would go like this: Hablo poco español. No it is not incorrect, but we usually omit pronouns in order to sound more natural.