Depending on how many sounds you want to output from the system at any one time you may have to look into things like having multiple filters, quad VCAs, quad Mixers, to get the proper I/O in your system, but if you only want to output 1 arp / sequenced sound at a time then you don't need that stuff.

I havn't got that far into eurorack yet, so I would either presume that some compressor modules come with a side chain signal input or you might have to use something like a envelope follower, im not quite sure.

Drums are tricky, you can make them from basically anything and layer as many things as you want, everyone has a different perception on drums.