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It is fine to sing after vocal exercises. Just don't try to substitute good vocal exercises with random songs. Vocal exercises are designed to grow your voice in an orderly and controlled way.
Songs can vary a lot, and may be beyond your present capability. So it's a good idea to stay with practice songs that are within your present abilities. It's OK to stretch your capability a bit, but don't go too far at once. Give your voice time to grow.