Stargazer lilies do not fit into the equation, as they like acidic soil and peonies slightly alkaline(according to real peony growers, and not random people). There will not be too much going on, as peony flowers are very short lasting, so they practically do not count.

I guess I'm a random person ;>) since I have both peonies and orienpet lilies growing well in the same bed where they've both been for years. My soil is slightly acidic, but I haven't had any issues with peonies, and I grow both tree and herbaceous plants.

It sounds like you are planning for pretty much constant bloom of one type or another. I think how many Thalictrum you use and whether you add columbine and Nepeta depends on your taste and the amount of space you have. Do you want more mass in your Thalictrums, or do you like the airier look of single plants? Do you want a more overflowing cottage garden look or a more pared down look? What colors are your various plants? If you are pretty much sticking to white and two other colors (probably pink and blue-purple from your plant list?) I don't think it would look too busy, but that's my view, and assumes enough space.

Thank you both! I have seen the lillies with peonies many times and apparently doing well but I wasn't aware that peonies needed more alkaline soil. I think i will give it a try and hope for the best! But thanks for the warning! :)

I plan on having two different bloom times for the peonies to extend it a little, eventhough it is still not a lot. I am hoping for constant bloom and the colors are the ones you mentioned. I definitely prefer the cottage garden look.

As for the space... The bed is approx. 26 feet long but only like 4 feet wide. So, once i put the big shrubs there might not be a lot of room. But i can still change the amount of plants. I have never seen Thalictrum personally. I just saw a close-up of peonies with it and fell in love. I researched it and thought the Hewitt's Double was lo prettiest. I love the contrast.

But since i have never seen it, i don't know how it looks better. Also, i read somewhere that it spreads a lot. Is it true?

And about the columbine, i just didn't know if it would go well having the Thalictrum too. I would prefer to have them all because of the different colors and blooming seasons.

Stargazer is not an Orienpet, it is a pure Oriental= acidic. Orienpets can grow anywhere, and will most probably push the orientals out of existence really soon. They already come in many colours which were reserved for pure Orientals until recently.