We had leylandi cut down on the advice of the surveyor when we moved here 16 years ago but have had no adverse effects only good ones.The roots no longer damage the drive and the rhododendrons have been spectacular every year so they weren't affected.

Yes, the soil will be fine after they are cut down. No toxin that I am aware of. Some trees and shrubs do use "chemical warfare" by secreting toxins into the soil but not Leylandii.

Long-term growth of Leylandis can impoverish the soil, however, so you may want to add some compost and slow acting fertilizer before planting.