I understand that कोई means someone. In other contexts it might mean a few or some. Is a valid to translate in this situation: 'Some are speaking Hindi' (implying that maybe some others are not speaking Hindi), or 'A few are speaking Hindi' (perhaps implying that there are not many)?
No. कोई refers to one person which is why it conjugates as singular.
Your example would be better translated as कुछ लोग हिंदी बोल रहे हैं।
That said, कोई can sometimes be idiomatically used in that sense. Eg: यहाँ कोई हाँ बोल रहा है तो कोई और ना। इन लोगों में समझौता मुश्किल है। (Someone here is saying yes and someone else is saying no. A compromise between these people is difficult). Literally, this sentence just conveys that there is one person on each side but may be understood to mean that the group is split between the two options. However, 'यहाँ कोई हाँ बोल रहा है।' would unambiguously mean that there is one person on the 'yes' side while the others are presumably saying 'no'.