" Is there a floor plan or layout of the train station in Brussel-Centraal/ Bruxelles-Central and the train station in Brussels-Zuid/ Bruxelles-Midi?"
Probably, but the layouts are fairly simple. They're both through-and-through stations. Tracks run parallel to each other, and they run in numerical sequence. Unlike most train stations, the tracks at Centraal are on the lowest floor, and the concourse area where you walk between tracks is one floor above. The next floor above this contains the ticket windows, the walkway to the metro, and street-level exits.
At Brussel Zuid/Midi, the tracks are raised above the main concourse, but like most large city rail stations, it's basically the same layout. The tracks all run in parallel sequence, and you move below to go from one to another.
Both stations have plenty of monitors to indicate which train leaves from which track, and a screen by each track will also indicate the train's final destination and everywhere it stops.
Don't overthink this too much. If you can navigate an airport, train stations are much easier. Just look at the monitors and follow the signs.