Morland was colonel commandant en second (Eugene was the colonel commandant and he was, in 1805, général de brigade)... and Morland was not general. L'Arc de Triomph is certainly a very nice source but there obviosly are not only generals (I don't really know the system of inscriptions, maybe Morland is an exception to honour him for his glorious and symbolic death at Austerlitz), and I would look rather to the Imperial Almanach of 1805 where you will find Morland to be "only" major, to the dictionnary of Six (where you will not find him) and to the dictionnay of Quintin (where you will find him and you will be said that he was colonel) :

"il est agé de 32 ans et compte pres de 13 ans de services lorsqu'il est promu major des chasseurs a cheval de la Garde 31 janvier 1804. Nommé colonel, commandant en second le régiment de chasseurs a cheval de la Garde impériale 9 juin 1805, il en prend le commandement effectif en l'absence du prince Eugene pendant la campagne de 1805 a la Grande Armée. Décédé a Brunn... le 5 décembre 1805... Son nom est inscrit sur l'Arc de Triomphe de l'Etoile coté est..."

We could cite also, for example, Berthier's relation de la bataille d'Austerlitz : "Le maréchal Bessieres dirige sa gauche sur deux escadrons de chasseurs a cheval commandés par le colonel Morland avec ordre de tomber sur la premiere ligne ennemie..."

The question of ranks within the Imperial Guard is not simple; in 1805 the regiments had 1 colonel commandant who was a general (général de brigade), and 1 major who could have been major or colonel and if he was colonel that he was named "colonel-major" as was Lepic in the grenadiers a cheval. Later the system became even more complicated.

Best regards

Jakub