Do the built-in adapters work with the newer Maxi-Cosi CabrioFix iSize car seat? If not, how can I find out which current car seats they do work with? I want to get the safest car seat that will fit the adapters.
It depends on which adapter you have on your Cosmo, as there are two that were sold and distributed with.
...will only fit BRITAX-ROMER car seats, even the newer 0+ ones. They are good car seats so I wouldn't worry, just they don't fit on all stroller frames.
With this type of attachment/adapters that can be attached to the frame in just the same way:
...you can fit a Maxi-Cosi, Cybex, Kiddy, Recaro, Joie, Nuna car seats to this chassis, so if you have that, you're all set. If you don't, as this is an older model, you'd need to browse classifieds and eBay, you might just find such (but I can't guarantee this).