A keckleon in the wild already noes thief.
I am pretty sure your Pokemon can learn it on a high level. :)
Only by Tm
yes it does.......around level 48
Ariados does not naturally learn the move Signal Beam in Pokemon Emerald. If bred with Venonat, Venomoth, or Volbeat however it will learn Signal Beam.
Gyarados cannot learn water gun.