Any plant you plant with bulbs should have a leafing pattern that arches over where you plant your daffs. That way you can hide the daff foliage while it dies back and the space wher they are planted. That is why daylilies are so good for that.

Plant your daffs in a group of 3 or 5 whatever you have room for. Dig one big hole and space them about 2 inches apart in a circle. 8 inches from that plant your daylily and another 8 inches from that another group of daffodils and so on. The first year the lilies won't cover the daffs but the next year they will. That year you can plant a spreading annual to cover the daffs. You must leave on daff foilage until it turns brown so the good goes back into the bulbs.

If you go for daylilies get the everblooming ones like the stella series or many happy returns.

Actually you don't have to go with lilies--I just think they make a good combo. I don't think Cranesbill will because it doesn't have the arching leaves to cover the daffs but there are other perennials out there that do you just have to look around.

I think it will make a very effective border. When the daffs are blooming the perennials will be sticking their heads out of the ground and when they are done you will have the perennials

I reread your post and I missed the 6 inch part. That is really not enough root room for a perennial. Perennials grow and it is not uncommon for them to be 12 inches across by the second year. You would be dividing them every year. It would be fine for the daffs if you planted 3 in a hole and then went to annuals.

Someone else might think differently.

It is true that 6 inches is very narrow to plant perennials, but it would work if you don't mind the perennials spilling over the edges of your soil. Some of the sanguineum hardy geraniums might work well there, but they would would definitely sprawl over the edges of the 6" strip. 'Canon Miles' is a fairly compact variety or even 'Alpenglow', 'Max Frei' etc. The sanguineums are all very tough, grow in sun or shade, and are drought tolerant. There are other rock garden types that are quite compact, but they are not quite as tough as the sanguineums and their hybrids. These include 'Ballerina','Lawrence Flatman','Carol', 'Alice', etc. Even with selecting daffodils, I think that I would pick the shorter, sturdy varieties such as 'Tete a tete', 'Jetfire', or 'Feb.Gold'. The other benefit of these somewhat smaller cultivars is that they will have less foliage to look messy after bloom time. Small bulbs like crocus or chionodoxa would look great in this strip, too. I think it is too narrow for daylilies or tall bearded iris. You could plant dwarf iris, but my personal opinion is that their bloom time is far too short to give them space in a small garden. Don't forget that you can still plant some annuals in between the perennials and/or bulbs.

I have cranesbill all over the place. I think it would work well to cover the drying daffy foliage, though you might need to do some tucking in at first. It tends to grow in dome-shaped clumps, with the leaves splaying upward and outward.
Like ontnative said, if you don't mind the plants hanging outside the 6 inch home then it will be fine.
Some cranesbill have longer bloom time than others - and there are so many varieties.
I'm getting excited now - next spring I plan on adding to my cranesbill collection. :D