Is it possible to hire one coach at a time? there doesn't seem to be a zero option when you click the 'hire new coaches' option.

You will have to "rehire" each time you change your coach line-up. Depending on how long you have played I would recommend build all your VIP seats first and then add a Fitness Centre and 20 Fitness coach. After that get all your Executive seats and then you might want to either get a big chunk of regular seats or Training Hall + all 20 coaches (expect kicking, since that isn't worth it).

Personally I am going for the regular seats first and then the full coaches and Training Hall.