I don't find thalictrums respond well to being messed about with. I only grow species so all will come true from seed and I let them seed
I only dead head thalictrums if I can see tiny buds which will produce more flowers. I only cut them back if they've been blown over by strong wind or if they start to look really untidy.
Like Nut - on the whole I tend to leave them to do their own thing and have found they self seed quite well. The seedlings don't mind being moved to a new spot in spring.
Thalictrums always tend to flop over once they have finished flowering. I used to cut all the stems right down and leave the foliage. It's the foliage that is of interest and a great foil for other plants later on. I don't see any problems with dividing up. Best to do it next spring.
collect and sow when ripe chrissie, don't save it, needs cold rather than heat to germinate.