Should my seated calf raise be as strong as my standing / donkey for the same number of reps?
Should my leg curls be as strong as extensions?
Would an imbalance create strength issues?
Nope, quads are bigger, therefore stronger by percentage
Yep, but its not in the numbers you lift that cause the deficiencies, well it is technically, but what I mean is, it is how much each muscle group is worked. Just don't neglect one lot of muscles over another lot. Try to balance your training and keep your body symmetrical and balanced.