I have no problem transplanting seedlings before true leaves open. I've been doing that with Eucalyptus maculata over the past few weeks. Smaller roots = easier to transplant without getting roots in knots = better root system on the resulting tree.
Also no problem waiting until the stems are stronger and new leaves open up. At that stage the roots will be longer. Young seedlings cope really well with root reduction so snipping long roots to make transplant easier is no problem.
I've also left seedlings in the community tray for a year, occasionally longer before transplanting but need to remember to fertilise diligently. Competition for nutrients, water and light usually produces undersize plants.
The seedlings pictured have long, skinny stems and all growing one way. They are clearly struggling for light. You may see high mortality rate if the light is not addressed soon.