Technician A says that there are a number of coolant types that each has its own life span. Technician B says that mixing of coolants is generally ok, since all coolants use the same chemical base. Who is correct?
Techn A is correct.. Tech B is correct in some instances, there are coolants that you can get away with mixing and there are cars that are more tolerant of coolant make up. But given the potential for undesirable interactions between different coolants and the subsequent potential for damage to the cooling system/engine as a result I'd be ignoring Tech B on this.
Technician A is correct. Technician B is NOT correct. Even if ALL coolants "used the same chemical base" (they do not), there are additives which could make them inappropriate.
For instance, certain coolants used in Ford diesels have anti-cavitation additives to prevent damage from tiny bubbles caused by the sharp vibrations of diesel operation. A large amount of coolant without this additive mixed in would potentially put the engine at risk - even if they had the same "chemical base".
This is most certainly an ASE type question, and I can assure you these questions should always be answered with absolutes. At least if you want to get the question correct according to ASE. There's hundreds of these questions, and if you study how the questions are asked, you can usually answer them correctly without the specific automotive knowledge:
Tachnician A [certified ASE reading the OEM manual] says (follow this rule).
Technican B [your uncle shadetree who heard something at the pub] (nah you don't have to do that if you don't feel like it).
Sometimes A and B are reversed, or both "professional", or both "shadetree" [neither A nor B correct] but you can always get a flavor on these questions if you read with one eye squinting...