To enforce a minumum stay for a rate it will suffice to associate an extra cost to this rate, even with a value of zero, that is applied always with a minimum number of days. This way you will get an error message every time you try to insert a reservation with this rate and a number of days below the minimum. It is also possible to enforce a minimum stay only in selected periods by using the option “always in peritted periods” when the cost is associated to the rate.