Purifying cysteine containing peptides
I dont know about HPLC column tolerance to various reducing agents, this would need to be checked. However, in biochemistry we routinely add reducing agents to avoid disulphide-bridge formation in the proteins.
Overall there are three main reducing agents used, to avoid this problem. The choice of which one to use depends on your needs. They include:

TCEP: Considered the golden standard, it is relatively odourless compared to the other reducing agents. Once you have dissolved it, it should also remain very stable over time. Typically used at a 0.4 mM concentration.

DTT: Cheaper than TCEP, although I have heard that it is not as stable over time

2-Mercaptoethanol: The absolute cheapest, stinks like crazy. Typically to a final of 5 mM.