The Mobile IronSource Collection is only available in the main collection. Unfortunately, I don't think it will change anytime soon.
There is a planned Google Admob Collection, which will be a cheaper variant, hence better for startups. But, in the long run I still recommend going to IronSource, due to their better mediation.
As for the separation of addons. There is an option to choose by group of addons you need, I don't think I can separate them further. This is due to the management it would require, and I don't think that will lessen the cost.
Bundling them together is more beneficial to both ends, in the long run.
There is also a discount for individual developers, if you are interested, it's at the bottom-most portion of the pricing page.
I wish you great luck with your game development journey!