If an antenna needs to be mounted on a metal structure what would be needed to avoid interference? Are there just some specific length to avoid or it is better to be separated by some insulator? As I am looking at towers, are there any parts that insulate antenna from the tower and each element? I can't see it from pictures only, what it looks like?
This depends a great deal on the type of antenna, the shape of the metal structure, and the radiation pattern you want. Ideally, you would want use the structure as part of the antenna, either as a ground plane, radiator, or reflector.
It usually isn't necessary to insulate the antenna from the structure -- this might even be dangerous, as you may need to ground the antenna to the structure for lightning purposes. If you are using an antenna designed for a tower, it is probably designed so that insulators are not necessary or are already part of the antenna.
You may need to put a spacer (usually metal, conducting) between the antenna and the structure so that reflections off the structure give you constructive interference rather than destructive interference.
It is possible in extreme cases with a tower to "tune" part of the tower to make it transparent to the frequency of interest, but usually if the radiation pattern is blocked by a tower too much, instead multiple antennas are placed on different sides of the tower to overcome this.
A specific answer to this question is not really possible without knowing the geometry of the antenna and tower and the frequency of interest.