This nexium protocol is useful for simple, uncomplicated gastric ulcer cases. It is supported by a small amount of literature and the many personal reports of success found in this thread and elsewhere.
While you can certainly choose to use nexium for complex ulcer cases in a new to you horse, I would not recommend that, particularly if hind gut ulcers are suspected. There is no actual research (that I am aware of, if anyone knows of a study, please share!) of how this particular PPI impacts the hind gut. We know that, in general, PPIs can cause harm to the hind gut.
My personal preference would be to scope a horse that is suspected of having both gastric and hind gut ulcers, preferably with an ultrasound of the colon as well. If gastric ulcers aren’t found, that simplifies treatment considerably. Also, true right dorsal colitis isn’t something to mess around with and can be challenging (at best) to treat. I would be very hesitant to put a RDC horse on ANY PPI. There are other options to heal gastric ulcers that aren’t so hard on the hind gut.
This horse does not sound like a good candidate for nexium @Satin Filly. Work with your vet, scope and ultrasound the horse, and question the use of PPIs all together if you do find he’s got any RDC.