Question

If the Andromeda and the milky way galaxy will combine , does it would cause any damages to the planets or starts inside the two galaxies?

Answer 1

Not really.
The galaxy is mostly gas and dust.
These things will combine and form new stars.
But it won't damage planets (unless the star forms near a sun).
But that's improbable, as the gravity of the star or planet will hamper the formation of new stars.

Answer 2

Yes , it will cause a lot of destruction if both the galaxy will combine

The combination of galaxy is actually a black hole merger (present at the centre of galaxy)

black hole merger is said to be one of the most energy emiting process in the universe
which synthesis harmful gama rays

also when two galaxies meet their planets and stars will bang against each other creating lot of destruction

also it led to creation of strange matter which is really "strange" having alot of harmful energies