Under expanding employee-rights law, you have like 90 days to make that determination. If you do let them go, make it clear that the problem is in the fit. They may well be a great employee for another company. When they do, both they and the company will be happy. You'll not be doing either them or your company any favors by letting mis-fits stay.
Work rules are important.
As in society, rules are important to protect the safety, comfort and serenity of individuals from each other. But they should be adult rules. But so are unplanned absences and tardiness. These cause unnecessary stress for those who have to pick up their workload and cover for them. Before freezing the handbook, talk over the rules with your employees.