Boy......that's a lot of questions to start out with but here goes :-))

First, hydrangeas are really shade loving plants or at least prefer part shade. Ideally, morning sun and afternoon shade is preferred so locating in a west exposure is really not recommended. Ditto with most hostas.

Spring blooming bulbs - like daffodils (narcissus) and tulips - love sun and good drainage. These are usually planted in fall for spring bloom but you can sometimes find started bulbs available in nurseries now for immediate spring planting. Iris also need sun. You can certainly mulch over the top of bulbs - they will grow right through it.

As to preping for a new garden, soil is the most important factor, followed closely by light conditions. I'd recommend removing or thoroughly killing off any grass before creating a new planting bed as any remaining roots left in place will just create weeding headaches down the road. Adding compost or other organic matter will help to improve any garden soil and can also improve drainage, which most plants require. I'd consider 2-4' to be the minimum planting depth for any new bed, as even most perennials will eventually spread to fill that space, not to mention any larger growing shrub. Obviously, if the hydrangea gets to be 5-6 tall and wide, a 2-4' planting bed is going to be far too small. A greater depth (6'+) will allow layering of different plants to create more visual interest and eliminate the 'soldier' effect of a long and narrow straight line of plants.

As to what else to add to any planting area, the list is endless!! You might want to visit your local plant nursery to see what they have available for planting now and it is also helpful to visit any local public gardens to see how they combine different plants. It is important to combine plants based on similar soil, light and watering needs, so do your research before purchasing/planting :-) The same research will give you general information on size and spacing as well. Lots of info available online or you can ask on any of the other plant-specific forums. Be sure to include plants that will remain intact - like woody shrubs and smaller evergreens - in winter months as well, otherwise you will be dealing with bare soil for 6 months of the year.

Gardening and gaining plant knowledge is an endless and always entertaining task, so read, read, read and read some more :-) It might also be helpful to purchase a basic gardening how-to book for general reference purposes and also one on home landscaping. Often, the home improvement stores like Lowes or HD will offer these very inexpensively in their book and magazine department.