You should try to tailor every letter to each salon, pinpointing reasons and to why you would like to work there.
E.G: If the salon uses Darphin products and does Pulsed Light treatments, you would want to pick up on these by saying you are really interesting in using natural products with essential oils and plant extracts (darphin) and you are hopefully looking to train to do Pulsed light treatments as you have heard of the fantastic results and have researched that brand and think it is very sucessful in its treatments.
Yous should show that you have something to offer to the salon apart from just turning up to work, doing your job and leaving. That would make me look twice at the cv and interview you.
Hope this helps
xx