Is it true the baby in Harry Potter 1 is albus severus in Harry Potter 7?

No. Albus was played by Arthur Bowen, who was already two years old when the first Harry Potter was filmed. In the first film, baby Harry was played by the Saunders triplets. Parts for very young children are often played by twins or triplets to avoid overworking them and their first names were not credited. However, it's obvious enough that Arthur Bowen's last name isn't "Saunders".