I would prune Cornus Kousa and Florida later in the year after flowering. The Alba Siberica I would prune now and very low. Best to prune about every 2 to 3 years to maintain the new coloured growth. Yes, you will lose the flowers but they're not that wonderful anyway. I prune my Sambucus hard in the Autumn at leaf drop and it doesn't seem to affect the flowering. With Buddleja you can prune now but the later that you prune will delay flowering, so if you have many plants then you can spread the flowering throughout the summer by pruning at different times.

I've pruned a 1/3 of my cornus 'midwinter fire' stems evenly through out the shrub, so that I have coloured stems and flowers. Then the new growth has lots of good colour in the winter. I pruned the buddleia last week and I will get flowers by June. I don't have sambucus but it can be pruned now if you wish.

Thank you for your answers. Have pruned Sambucus and buddleia today. Also sowed sunflowers, zinnias, cosmos and calendula. Got a bit of catching up to do......

I pruned my sambucus earlier this week