I agree that it’s potentially tricky, if you don’t set clear boundaries beforehand.
I’m going to make some assumptions here which might be entirely wrong because obviously I don’t know you or your set up but these are very common issues that get posted on Salon Geek regularly.
“I can already see her being used for their columns”
Is this a planned part of widening the assistant’s experience? (In control)
Or do you struggle with assertiveness and boundary setting? (Not in control)
As you are using chair renters in the salon, do you have any experience of managing staff?
You will need to be supervising the assistant and organising her daily work schedule to ensure she’s actually encouraged to learn and practise and not just cleaning and helping you all the time.
I think you should keep her working as your assistant only and only occasionally assisting one of them if you direct her to when you’re not needing her help. Be very clear that she only reports to you.
As the other stylists are self employed, you could charge them a higher rental fee to include having the use of the assistant, but if they couldn’t use her services when they wanted to, they could argue that you were in breach of contract for not making her available to each one of them, all the time. (It would be like you sharing a cleaner with 2 other businesses and paying the same price as them but the cleaner spends 3 days at business 2 and you only get him once a week. You would be annoyed that you’re effectively subsidising business 2, and so it wouldn’t really work.)
I think you should have a meeting with your chair renters explaining your plan to take on an assistant and make it clear that they won’t be covering her wages. However, she might help them out occasionally but entirely at your discretion so they cannot treat her as their employee. They have to ask you first, every time. You’d also have to make this very clear to the assistant so she doesn’t think she gets to pick and choose what work she’s doing.
Also, regularly review the situation talking to both the assistant and the chair renters individually to ensure that the arrangement is working well.