can you grow from seed Phormium's and cordylines ?

Yes you can grow them from seed. I have grown phormiums from seed. Treat them as any other seed, but they do grow very slowly to start with. always better to divide what you already have to get quicker results.

Cordylines are easy from seed too. Almost all the large Cordylines in my area of south east Northumberland of any size seem to have flowered. Tiny white berries will stud these branches sprays over time and each is filled with two or three hard black seeds. They come up like mustard and cress (or grass in this case!).