"He is between a cat and a dog"
Translation:Yupo kati ya paka na mbwa
Yu- is equivalent to a- used with verbs, but is used with the locatives po/ko/mo. In standard Swahili, you can't say apo - it is yupo instead. (This yu- is a funny little quirk, but it can also be seen in the demonstratives huyu and yule.)
-po indicates a fairly exact location. If the cat and dog were very far apart and your intention was to say "somewhere between ..." then I'd guess (but I'm not certain) that yuko would also be possible.
Both should be accepted, and both can be used to mean something else in a different situation. Kati can mean "among" for example: Kuna msaliti kati yao meaning there is a traitor among them. Katikati can mean in the "middle" for example: Usicheze katikati ya barabara meaning Do not play in the middle of the road.
No, that is not correct. You can say "Yeye yuko kati ya paka na mbwa" or "Yeye yupo kati ya paka na mbwa" but you really don't need that "yeye". You can use a specific name such as "Juma yuko kati ya paka na mbwa" or "Juma yupo kati ya paka na mbwa." And, when it comes to yuko and yupo regular Kiswahili speakers usually use those two words interchangeably.