Mathieu,
There is no single official source even suggesting that another unit than KG54 nor Ju88 were involved in the bombing of Rotterdam.
On top of that:
- throughout the first week of the Westfeldzug there is no single example that a target was attacked simultaneously by two different Geschwadern. As far as I can see the Luftwaffe had a strict policy of one unit per target, almost certainly to avoid complex joint planning excercises. I suppose planning for the right fighter escort at the right time was already difficult enough.
- the only bomber units operating over The Netherlands those days and equipped with Ju88's were KG30 and III./KG4. The first was (after May 10) uniquely used for anti-shipping and the second did not take part in the bombing of Rotterdam.
- the Ju88-equipped III./KG4 did bomb on May 14: in the evening Den Helder was seriously raided, after the official start of the cease fire
Regards, Pieter