high voltage insulator strings

Each type of insulator has advantages and disadvantages but it certainly isn't necessary (nor common) to mix different materials or strength ratings within a given string. It may occasionally be done when damaged insulators are replaced in a string, for example, and matching insulators are not available. That's not to say that somebody might not apply polymer "shed extenders" or some other devices, or even mix insulator types in order to improve voltage grading, contamination performance, eliminate line end cap or pinhole corona (which can be improved by using higher capacitance units on the line end of the string) or, for some other reason. There is very little one can do to improve the insulator electrical performance other than to apply grading rings or other devices to obtain more uniform voltage grading on the string or individual insulator in the case of polymers.

You've mentioned the only information that can be gained from measuring the voltage distribution on the string.

In the final analysis, air is the primary insulation on a high voltage transmission line (or any other overhead line). The primary purpose of the insulators is to support the line mechanically and not reduce the dielectric strength of the air gap under any condition.

For example, if overhead lines were built in clean, dry, pristine environments, one could satisfactorily insulate with a wooden broomstick; providing the broomstick had sufficient mechanical strength to support the line mechanically.

The first part of the answer is correct.It is not common to mix different insulators and form a string.When u say HT I take it as voltages 110KV and above.The insulator string provides clearance to the cross arm of the tower from the live conductor as the tower is grounded and accidental contact or any contact can result in a Phase to ground fault.The insulator string has to met the BIL set forth in The standards.The length of the string depends upon the Voltage of Transmision, span between towers,Swing due to wind and other parameters that may be applicable for a particular location.There are again two types of strings (a suspension type and a tension type)used a vertical one(suspension Type) holding conductor at the bottom and a horizotal one(Tension Type) .The second is mechanically more strong as it takes the load (50%)between two tension towers.In tension tower the conductor is terminated and by a jumper connection is continued to other side.In railway crossings they some times use a double string so even if one string fails other will hold .Suspension towers only support the conductor and are cheaper.More of this can be read from various books/hand books.Air is the insulator between conductors of the same circuit and also between two circuits if the tower is a double circuit tower.By Voltage testing u can determine whether a string is healthy or not but rarely done in the field as any failure will be reflected by the protection.The insulator has to take electrical and mechanical stresses that it is subjected to.hope this meets some of your need