You may need to do a bit of hunting, as a lot of the group 3's with bell-shaped flowers are viticellas or viticella hybrids (and a few other species clematis) and have a tendency to grow quite a bit more robustly than 6'. One to consider is 'Arabella', which is not a vining type of clem but will grow through the support of smaller roses and shrubs. A rather pale mauvey-blue flower that combines well with more intense pinks or reds. Some of the newer Evison intros may work well also (i.e., 'Gazelle'), as they tend not get overly tall and many are intended to be treated as group 3 and can feature the flower shape you are seeking.

Thank you, Arabella is a very nice choice. I was also considering Crispa but I don't know much about it.