All the branded medicines (Aricept, Exelon, Reminyl and Ebixa) mentioned by you are meant for the treatment of mild to moderately severe dementia of the Alzheimer's type. They have not been tested in other varieties of dementia.
Dementia is treated symptomatically since there is no known cure. The symptoms may be agitation, loss of sleep, apathy, urinary tract infection, visual hallucinations, delirium etc. If any of these symptoms appear, appropriate medicine is to be given.
Non-drug behavioural therapy is also equally important. For example for lapse of memory, note books should be given. Walks should be included in the daily routine.
Many patients of dementia also concurrently suffer from depression that needs to be treated.