There is no service interval for the power steering fluid and filter.
There is no service interval for the differential fluid.
YES you can ask for the services seperately if that's all you want done.
It may not be the best, you may have other work that needs to be done, heck, they might even look around and try to upsell the extra work anyways, but don't come crying to the tech or shop manager if you drive it 2 months and the brake pad warning light is on, your muffler falls off, and the serpentine belt does an adios
at midnight during a thunderstorm. But sure, you should be able to get these services done seperately, sounds like a GOOD IDEA to me!