Due to their being electrically the same (aside from Eurorack’s rarely used +5 V rail and CV and gate distribution), Kosmo and Eurorack modules work well together. Both can be powered from the same power supply using the same ribbon cables. Signals produced by one can be used unmodified by the other. The one wrinkle is the patch cord sizes, but 1/4" to 3.5 mm adapters are available, or you can use a “jumbler” module with both types of jacks connected together to go from one standard to the other.
A bit less obviously: There are literally thousands of Eurorack modules in existence compared to literally dozens of Kosmo ones. So Kosmo systems are inherently more limited than Eurorack ones? Not really, because you literally can take a Eurorack PCB and mount it behind a Kosmo panel. You would need to make your own panel of course, and to do something like wire panel mounted 1/4" jacks to the 3.5 mm jack footprints on the PCB, and to get Kosmo-style wider spacing for bigger knobs you may need to wire panel mounted pots to the closely-spaced pot footprints, and having done that you may need to kludge up some new way of mounting the PCB to the panel. But it can be done. If a Eurorack PCB is available, then a Kosmo PCB is available too.
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Kosmo format modular synthesizers