Is it legal for a Company to make it mandatory for staffs to use a specific Credit Card?

Yes it is legal and completely normal for a company to require that associates use a corporate credit card for travelling, fuel reimbursements, meals, and other business related expenses. If you want to be reimbursed you will use the card dictated by the company. Individuals that abuse the corporate credit card will find that the company will not reimburse them, and their spending privileges will be revoked.