Okayama station

I'm planning on travelling from Tokyo to Kyushu, hyperdia gives me a timetable where I'd have only 5 minutes to change trains in Okayama. Would that be enough ? I'm used to travel by train in Switzerland, I'm not so sure about Japan. If not, would that be worth making a stop there to visit Korakuen garden ? How much time would I need ? Are there big coin locker at the station where I could leave my luggage ?

hyperdia gives me a timetable where I'd have only 5 minutes to change trains in Okayama. Would that be enough ?

Yes, the connecting train will depart from the same platform (possibly opposite tracks) as the arriving train.

If not, would that be worth making a stop there to visit Korakuen garden ? How much time would I need ?

I would calculate at least two hours.

Are there big coin locker at the station where I could leave my luggage ?

Yes:
http://www.japan-guide.com/e/e2274.html

Will there be a food shop available for a quick purchase of rice roll with salmon inside? I love those.
How would we know which platform to go to? Would our ticket have that info already or would we have to check the tv screen again upon transfer?

There is a kiosk or shop with snacks on or near the platform. Not sure whether they have the specific snack you are looking for.

Like in Switzerland, train details are written on displays on the platform and elsewhere in the station and on the trains, themselves.