This is a terrible idea. I agree that it would reward training multi-skilled guys, but in reality a team can effectively develop only 2 top level players in 6-7 seasons. Rest has to be bought, so the teams that played longer or were tanking right before new rule was implemented would have advantage because they would be the ones grabbing most salary-efficient players, or be able to afford luxury tax.
Getting GS affected by a transfer is a great idea and now just need to manipulate how much the effect should be and tweak with it after testing
The GS hit probably will alter some strategies but in many cases I don't think it will make any difference.
As I said initially, I like the thinking of a salary cap I just think it would be impossible to implement fairly across different divisions and different countries.