Tree planting

If you consider the size of these in 30 years, then that is far too close together, but then the question is whether they will all survive that long, and what effect you are wishing to achieve. We have a copper beech that was a mere 5ft sapling 40 yrs ago which previous owner had planted 6ft from the boundary, it's now 40ft tall (having been reduced) and is 50ft wide and is in completely wrong place, but I love her. Similarly a horse chestnut which was 6 inches in a pot, is now 50ft tall and a pain in the @rse. Horses for courses. These trees in a garden only 30ft wide were perhaps not the wisest decision.

So in answer to your question I would intersperse the smaller or shorter lived between the taller and long lived to whatever spacing you want because whatever you do you will wish you'd done different. Not a lot of help, eh?