hi, vdixit. i think you still should check the soil in the rose bed ( drainage is the most important thing) if they look healthy, i guess it's not something to worry about. roses are pretty demanding - they are very heavy feeders.

assuming the roses are in pretty good soil, that still doesn't guaranteed wheather the whole property is in good soil. probably the previous owner just worked on the soil on the rose bed. if you creating the new bed, i know it's really hot out but you do have to work on the soil by loosening and mix in lots of compost first to improve drainage and add slow release nutritions , feed microbs. if you do this hard work, your plants, whatever you wish to grow will benefit from it and you'll realize it's the single most important, critical step you must take. and this will save you alot of problem in the future.

you can plant whatever you want now. if they don't bloom this year they'll bloom next year. if you go to nursery they'll have summer/fall blommers now, so you can do that now and leave a room for the spring bloomers next year then you'll have a garden that blooms throughout the whole year . one thing that you have to remember is that since it is in the middle of summer remember to water well , the new plants wilt easily in general and the high temperature will make it even easier. mulching will be very helpful for water conservation and cooling the root and it'll definatly help tocut some of the transplant shock.
so enjoy your new garden.

whenever they sell it in the store,it's time to plant it.
here is a good link for gardening in seattle.

Here is a link that might be useful: rainyside