The source explicitly says the following forms of Iqtida can be valid:
1. A person with Wudu behind an Imam who validly used Tayammum.
2. A person who washed the feet behind an Imam who validly wiped over khuffs.
3. A standing worshipper behind a seated Imam.
4. A person with normal posture behind a hunchbacked Imam.
5. A person praying by gestures behind another person praying by gestures of the same valid type.
6. A Nafl worshipper behind an Imam performing Fard.
These examples prevent the rule 'the Imam must never differ from the follower in any respect' from being overgeneralized.