In Civ 1/2 you definitely DID NOT kill all stack of units if you killed one. I played them both a lot (and actually prefer them to newer installments), so I know for sure.
On more difficult AI levels (Prince or Emperor), the AI would meet you in the field, and you'd have to worry about stack kills. And also worry about cheating AIs who just had more resources than you...
Now, thinking of this further, i think I might misremember this because I've never let my stacks be attacked in the field.
Best tactics is to drive tanks via railroads up to the city (sometimes building them in the fly using a ton of settlers in one move) and attack it with 20+ tanks and then move in 10 mech infantry for defence, and I used fortresses extensively.