You need to be very careful planting rhododendrons because of their rampant spread unless you choose your varieties carefully, and because of their toxicity to livestock.
This would be a wonderful place to have a wildlife corner. Yes leylandii contain a growth inhibitor, and remove all the moisture from the soil. At this time of year rain will deal with the dryness, and a good load of muck would help the soil. Planting well grown shrubs and flowers will give them a head start, but choose sturdy, strong, unfussy varieties.
You haven't said how big the triangle is. Perhaps a few native hedge plants such as hawthorn could provide a basis for your wildlife corner, with wild roses, rowan, elder, hazel, guelder rose, something dense and evergreen such as holly to provide roosting for birds. You would also have hips, haws, elder and holly berries for the birds in winter.
Your understory would have foxgloves (unless livestock could browse on them), bluebells, primroses, red campion, toadflax, elecampane, avens, wood anemone, teasel and more similar woodland edge wildflowers, depending on your location (I'm a bit out of touch with wild flowers as we have so few round here). Most wildflowers don't like too rich a soilbase, but in this case you would only have restored the land after the leylandii were there. Once you've set up something like that, there is very little maintenance needed, maybe checking the trunk protectors while the trees are young, trimming broken branches and so on.
Topic: What to plant? (Read 3118 times)
