620 film and 120 film is exactly the same film, The only difference is the shape of the spool it is wound on. No special concern about loading on processing reels.
Many, many people choose to develop vintage film in Kodak HC-110 dilution B, as this developer is very clean working and minimizes age fog. I think Ilford makes an equivalent. This comes as a liquid concentrate that has an amazing shelf life (years) even in open bottles. You use HC-110 as a single-use solution, mixing up a working solution for each tank of film you process.