Answer: Generally speaking, ideally you would need to run some basic soil tests to determine what types of nutrients are lacking in the soil and then fertilize on the basis of the test results. Most plants do fine however with a general purpose granular fertilizer such as 10-10-10 spread according to the label directions once a year in the spring. You could also give them an annual top dressing of good quality compost. Using an organic mulch also helps feed the soil gradually over time as it rots down. Mature plants often do not need any fertilizer, especially if they are growing adjacent to a lawn that is fertilized. It is better to underfertilize than overfertilize, so if in doubt, don't.
The perennials you mention can be trimmed off short in late fall after freezing weather has killed the stems and foliage, or you can wait and do it in early spring. It is a matter of personal preference. For example, some people like the look of the seed heads on the rudbeckia and grasses in the winter time and the birds may also enjoy the seeds.
I should mention I would not expect your purple fountain grass (Pennisetum setaceum 'Rubrum') to survive the winter. It is only hardy to zone 8.
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