When insulin treatment is first begun, the veterinarian usually does an insulin- glucose-response curve to determine what type of insulin would be best to use, how much insulin is needed, when and often to give it, and when to feed. These are variables that are individual and often related to the size and metabolism of the pet. NPH is intermediate. Usually one is chosen. If there is a problem in getting good regulation then a different type is used. I think it would be very difficult to mix types.
You might also want to consider DIA-IONX to reduce the symptoms of diabetes. I try to improve the terrain of the whole body by also including probiotics and Omega 3,6,9. There are other supplements too but should be introduced gradually. Please let us know how you do.