Should I use 'also', 'plus', 'in addition', or 'besides' here?
Because ni3 and hen3 are both the 3rd tones, people usually say ni2 hen3 rather than ni3 hen3 in spoken Chinese. Also/Plus/In addition/Besides, mang4 is incorrect. You should say mang2.

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For some reason it's difficult to answer these kinds of questions, even though I'm fluent in English. :P

I'd say either "also" or "in addition" would work fine. "In addition" sounds more professional I think.

"Besides" is just incorrect.
"Plus" just sounds bad.

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The first sentence is fine, however the second sentence sounds a bit awkward. I'm not able to discern the relationship it holds with the first sentence. It's not clear if you're talking about the fourth tone or if you're talking specifically and exclusively about 'mang4'.

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