Well generally you can prune any group to some extend, just that you have to see when to do it, if clematis blooms on old wood, then better to do it after spring bloom. The main difference for me mostly is that group 3 usually is less trouble if you neglect it for a year, since you just cut it back in spring, when the other groups can be rather annoying toothpicking, especially if it gets between the other plants. Also, you do not have to necessarily cut it back to a few inches even if it is in group 3, it is more about where you want the blooms to be. This group clematis blooms on new growth, so if you cut it lets say at 1 meter high, all the flowers will be higher up than that. It is useful if you want clematis to bloom above your shrub roses for example. In general though you have to keep in mind that clematis attaches itself with the leafstalks to the other plants and structures, so usually it is annoying to get it out of the rose no matter what pruning group it belongs to (unless you prune the rose or don't care about leftover wood). I prefer to have roses and clematis near each other, but so that only few canes/liannas mix, or plant together varieties that do not need annual pruning, just some tidying up.

I don't have any specific suggestions, just a couple of general things to think about. To help you find a clematis that won't totally overgrow and overwhelm your bird house (such as sweet autumn clematis,) you can search by plant height (in meters) on the search option on Clematis on the Web. Also check the group; Integrifolia group plants don't climb well and so would need help to weave into a support. I would think that regular fussing would be disturbing to the birds, so you probably want to avoid an Integrifolia. As far as support, I have seen others put green coated wire fencing around posts, fastened loosely, for clematis to climb. My favorite clematis are the summer-blooming group 3s since their bloom time tends to be a when fewer other plants are blooming, and since they bloom on new wood, I have fewer worries about hardiness of the vines and buds (but that is less likely to be a problem for you.) They also don't get wilt, while some of the group 2s seem more vulnerable to that. Here is a link that might be useful: Clematis on the Web

Pruning group 1 are all species clematis and purple is not really a color option with these. You can get some nice bluey/lavenders with the alpinas or macropetalas however. I would recommend a pruning group 3 vine. These prefer to be hard pruned annually, which I find easiest to deal with when growing through or on other plants. 'Jackmanii', 'Gypsy Queen' or 'Etoile Violette' are all great purple flowered varieties. Try and avoid watering on any foliage. It promotes all manner of diseases, especially with roses. You may have manually adjust your sprinkler head to accomplish this.

I had several Clematis when I lived in Michigan. What I couldn't figure out was why some would flourish and some would start to grow and then die off. I believed it had to do with how much light they were getting. You may want to keep that in mind. I do know that the base of the plant doesn't like direct heat from what I read. I always covered them with wood chips. They are beautiful and I really enjoyed having them. I am planning to plant one at my new house here in NC this year too. You may want to view different opinions on prunning when they die off too. I used to cut all the dead vine off but I would wait until it started blooming in the spring because Clematis stars re-growing on itself at random places. I had friends tell me they would chop it off at the base and let it start from the ground up. Mine were always full of blooms and really thick and hardy.