I calculate that given the box type I choose right? (if I choose rhombic dod and I need 20% AA I just need to make calculations, correct me if I'm wrong)

Not only the box type. The -d option of gmx editconf changes the box size as well. The stdout output from gmx editconf shows you the box size, volume, diameter, etc.

that's gmx pdb2gmx, so I assume I must find a .pdb of my co-solvent single molecule

Yes, just run gmx pdb2gmx. Remember to save the acetic acid .itp file produced here. The default output name is posre.itp but you can change it using the -i flag.

Edit your topology to #include the appropriate .itp files, as well as make changes to the [ molecules ] directive to account for all the species in your system.

You need to include the .itp file for acetic acid in your topol.top file. Something like that should work:

; Include acetic acid topology
#include "aceticacid.itp"

The position of this line in your topol.top is important. You probably need to put it right after or before the lines that include the water topology (; Include water topology, etc).

You also need to add a line at the [ molecules ] part of your topol.top. It's in the end of topol.top. Example:

[ molecules ]
; Compound        #mols
Protein_chain_A    1
SOL             1000
ACH              200

If 'ACH' is the name for acetic acid in your original pdb file.

GROMACS 5.1.2 split gmx genbox into gmx solvate and gmx insert-molecules. I would probably first use gmx insert-molecules to add the acetic acid and the run gmx solvate to fill the remaning of the box with water.