Potting soil can be a limiting factor to good growth of plants and various mixes have been tried for the seedling stage. Any mix with salt in it should be avoided and I have had sub-optimal results using unwashed coco fibre, regardless of what it has been mixed with. Better results were obtained when using peatmoss (sphagnum moss) mixed with matured small pine bark chips, however there should always be good air movement inside the pot. I normally fill rooting bags with a mix of spagum moss and Perlite and then water well for a week or so prior to use. Compost can be of widely different quality, some are cheap and nasty and with others the Ph may be too high as most Proteaceae live in a low ph environment of around 5.5, and lower.
When potting up from the 70 ml cells into 500 ml or 1 litre pots, I use white yogurt containers (ex-supermarket) with holes burnt in the bottom and sides using a soldering iron.
The white containers reflect the sun and keep the roots appreciably cooler and side holes allow greater air circulation. A thermometer inside on the sunward side and another on the lee side proved this conclusively when compared to black containers. Roots will migrate from the hotter side to the cooler, so rotating the containers will set back roots on the new sunny side. Recycled yoghurt containers last for two or three seasons before they become brittle. It is good practice to give the used containers a bath in sodium hypochlorite (bleach), or a disinfectant, just before re-using.
It is my wish that tall square white pots with four vertical inside ribs with side ventilation become available. At present these are only available in black but painting them white does not work well as the paint flakes off fairly soon.