A VCA can be used to attenuate the signal. But that's just one signal. With a mixer, I assume you will have multiple channels of Eurorack level audio. So you'd need an attenuator for each signal. Six signals would mean six attenuators. If you're using a VCA for each, that's six VCAs. Which is expensive for just simple passive attenuation. Your mixer may have enough headroom without it and then you can turn down the input trim if possible. It really depends on the design of your mixer.
I wouldn't plug Eurorack levels directly to a powered speaker. You might end up blowing it up with high levels overloading or high levels of DC current being powered through the amps if the speaker isn't guarded against DC.
You can always conservatively try out the inputs by using an attenuator pot'ed all the way down then bringing up the levels while carefully listening and watching the meters on the mixer. But I wouldn't do something like taking the raw output of an oscillator and plugging it into the mixer.