|Quaking Aspen is Populus tremuloides. If the trees you planted did not have the right leaves, they probably were not Quaking Aspen, but some other cultivar. Quaking Aspen trees are planted extensively in the greenbelt at the Seattle Art Museum, so I expect you should be able to grow them in your Seattle area landscape. You might want to visit SAM and look at the trees to make sure that's what you want. Aside from watering them regularly, Quaking Aspen won't need any special care.
Best wishes with your new trees!