Can my students sit Cambridge IGCSE Physics 0625 with Cambridge IGCSE Combined Science 0653?

No, not in the same series of exams. In a single series of exams some combination of syllabuses are not allowed and you should refer to section 5 ‘What else you need to know’ in the Physics 0625 syllabus to find out the exceptions when combining IGCSE Physics 0625 with other Cambridge International syllabuses.