Hi Steve
I am no expert but this might get the ball rolling.
Tanto around 30 cm
Wakizashi Between 30 to 60 cm
Katana 60 cm and over
So broadly speaking they are length terms.
Shin Gunto Shin means New, Gunto means Military sword. These are usually non traditionally made swords and are not true Nihonto
The curavture of the swords suggest they were used for slashing and all were used for self defense at least. The WW2 swords were mostly used to boost moral I have been told. A bit like a bagpiper walking through the fileds of Flanders
In the Edo period, the katana and wakizashi combination, known as Dai Sho, became a status symbol of the samurai.
The masses of the Imperial Japanese Military who were eligilble to wear a sword (sergeant or above I think) would use a Shin Gunto. The more wealthy (usually officers) would use a true Nihonto in WW2 mountings. The navy generally were fitted with tanto blades and the miltary, katana. Also machine operators like pilots and tank drivers might have a wakazashi because they were shorter.
To remove rust. Be careful as it might be best left alone. You could try something hard but softer than the bayonets materials. Bone usually works but again be careful as you could destroy the something.