Much as Rangers are the club in Scottish football that I most dislike...
I do think that if the 12 /12 / 18 proposal is adopted, it should be for season 2014 / 15 rather than next season.
If it is for next season, it will be decided so late on that they are going to have to make decisions for how the two extra teams get into the second tier and decide what if anything ( Rangers are assuming they get nothing ) happens to the winners of the current fourth tier, while being aware of the teams that look in the key positions in the divisions. In particular while I believe there is a case for the 3rd division winners at least getting into a play-off for the new second tier, the fact that that will clearly be Rangers will influence very many for and against that idea.
I would next season leave promotion between the Premier and 1st division unchanged, then to increase the 1st Division to 12 teams :
Top 9 teams in 1st Division retain their status.
Top 2 teams in 2nd Division promoted.
Play-off for final place between : the bottom team in the 1st division, the third and fourth teams in the 2nd division and the top team in the 3rd division.
You could do this for the end of this season I guess, but as I said, many would think / be concerned it was a sop to Rangers rather than something for topping a season long league. Probably the most entertaining thing actually would be to allow Rangers into a play-off and for them to miss out

And also it could effect some others' fates based on how precisely they arrange any prmotion and play-off criteria.
While yes, Rangers would still be in the third tier of Scottish football whatever, it is indeed pretty ridiculous that a team can top a league for no reward over all the teams they have played in a season and then end up playing all these teams again the following season plus some more.