New residue top

can you explain “introduce by analogy”?
am I correct in understanding I can build the RTP from cys glu and gly along with adding for the S-S atoms which I can find in the atomtypes file?
then if I used the atom types of traditional AA i should only have at most a few dihedrals to address which I can write by looking through the ffbonded file and finding a similar set of bonds applying my specific atom types to add to my missing dihedrals.itp

Look at existing residues and assign the same atom types and charges for chemically equivalent (or sufficiently similar moieties).

Yes.

Correct.