Hi,
Is the usage "Warmest Regards" Correct ? Yes. Does it has any restrictions for usage ?
Where I live, it is not common in business correspondence.
You can use it in personal writing if you want to.
To my British ears, it sounds like a slightly odd mixture of emotion and formality, so I don't use it myself. It's too emotional for people I am not very close to, and too carefully worded for people that I am very close to.
Clive