Is it permissible to keep employees who are not currently needed and pay their salaries from Zakat funds, and is it a condition to inform them that what they are being paid is from Zakat?

If the worker is wealthy and able to earn and work, it is not permissible to give him from Zakat; because there is no share in it for the wealthy or for the strong who can earn. Similarly, it is not permissible to give Zakat to someone who is owed wages by you for his work; because that would combine two obligations (wages and Zakat) into one. However, if they are dismissed and cannot find work to sustain themselves, there is no objection to giving them Zakat as long as they are eligible for it.