Should I upgrade my generals(which skill) or get a new one?

Given a new update is coming soon, you should hold off on all medal spendings.

1. Wait for update.
2. You have a good lineup for starting. Don't upgrade Messe. You will probably want to exchange him as you progréss.
3. Kutznetov is a very good general and might be needed early but, like Messe, you will want to exchange him for Donitz. Donitz is the best pure Navy commander and can solo most naval opposition post upgrades. Since he is in the thick, i give him sailor and rumor or explosives, but thats why i still have Nimitz with explosives and rumor and havent exchanged him yet, so i havent upgrade him much either.
4. Yamamoto is who I also keep because he can replace Arnold and Nimitz, though i still have Nimitz also. I gave him Rumor, cover and upgrade an arty star to max so i could use him on land or sea, replacing Arnold.

For Warzones, you will want as many commanders as possible as you can't re-use same gens. Over and over in same warzone. And you'll learn that hybrid gens like Graziani,Yamamoto and Montgomery can be of big help in these.
Because of the warzones i use 5 gens for each troop type Inf., Artillery, and Armor, except Navy where i use Donitz solo for main navy battle. Yamamoto will go as Arty/air general there. Monty as Tank/air. Graziani infantry/tank.

Tanks~ Manstein Guderian Rokosovsky Monty Rommel (can be used on arty)
Infantry ~ Rundstedt, Yamashita, MacArthur, Graziani, Nimitz
Artillery ~ Konev, Leeb, Zhang, Bradley, Yamamoto.
Navy ~ Donitz.

I have the silver medalas and switch them to any general i use per warzone, making one in each type, a benefactor.
Example. Yamamoto has no skills which have medals so i could use the Excellence shoot again medal, Air medal, and artillery Legend which dont need specific skills to activate.