Obviously the thinking behind colts teams within league is well intentioned assuming you agree it would help young players develop. I can understand the arguement it would distort competition at the lower levels so maybe 6 would be too many. Even it was just 2 a season competing in the 3rd tier and theyd be replaced every season with the next top 2 from development league. Maybe this would give the development league a more competitive edge with the aim of getting to play a season against league sides. Changing both teams every season would mean no colt team would be promoted then, it would just be a 3rd tier experiment.
I do really like the top 16 idea above all other suggestions though. I just think we're too small a nation not to include decent sized clubs (St Mirren, Dundee United, Falkirk etc) in the top tier. Young players within these teams would surely benefit too from playing against stronger sides. As for the 30 game season being too short, you could still do a split whether it be 8/8 or 7/9 even.