Hi, how much sun does the area get? If a lot of sun, then don't use hydrangeas. Juniper does a great job holding a hill that size, but I'm sorry you don't care for that look. How far apart are the roses? You might not need anything between them if you have enough of them. How about several trees? Red maple 'October Glory' would provide a lot of root mass to hold the area. You can get those at most any Home Depot/Lowes.

The best mulch for a slope is coarse hardwood mulch. Pine bark nuggets, by the way, is the worst. With coarsely chopped mulch, the pieces knit together. But keep in mind, this was a really bad rain and sometimes nothing can withstand that. Most of us lost a LOT of mulch in this rain.

It is hard to get a new landscape started. Patience is important - and fingers crossed that we don't have that kind rain again for 5 years or more!! Good luck.

There's a link to a good article, below.
Esh is right when suggesting low growing, matting plants with webs of roots (such as juniper). That is the difference between say, juniper and hydrangeas and roses - the junipers cover ground, the hydrangeas and roses do not - neither are well suited to hillsides unless you use a low growing landscape rose which covers ground because it roots along as it grows low. I have not found any low growing landscape roses that succeed for a location such as this - most get disease here without spraying quite a bit.
There are however, many other great plants - many of them native too. How about Rhus "Grow Low" http://www.entomology.cornell.edu/Extension/Woodys/CUGroundCoverSite/Rhus%20aromatica.html A plant that Esh suggested to me (and that I hope to use). There is itea if the ground holds some moisture and also dwarf (there is a very dwarf) Wax Myrtle. I think that if you planted some low growing groundcovers you could put some roses in. There would be something holding the soil in around there feet this way.
There are a few good groundcovers for sun - I like creeping raspberry because it is mowable. Phlox is good too, but you have to trim it up after bloom to keep it looking nice after bloom.

I'm sure others will have some great suggestions.

Here is a link that might be useful: Walter Reeves, Taming Landscape Errosion