If you want to turn your transients to gates, a logic module for each drum sound would be helpful. Once the audio goes above a certain voltage, you'll generate a gate through the logic module... just tap the "or" output. From there you can apply whatever you'd like. Envelope followers were also mentioned. That will generate CV based on the instantaneous amplitude of the signal.
And a third device would work if the solution is to simply open/close a gate: a comparator. These also play nice with logic gates to allow strange polyrhythms and weird timing manipulations. But Ronin's on point when he says...
Honestly, Eurorack isn't the best solution. You'd be better off with a laptop with an 8-in and 8-out interface and software like Ableton. From there you can process, envelope follow, and whatever the heck you want out of your inputs. But using Eurorack is just going to be a cluster with meager results.
Eurorack ain't for everything.
-- Ronin1973
And I agree 100% there. This isn't really the right application for Eurorack, as such. It might make more sense to use an electronic drum kit and process its output mix through a Eurorack system, thereby avoiding the need for the piezos, the extra modules, etc etc etc. But doing this purely in Eurorack will get expensive and unwieldy pretty quickly.