Armel,
VUM is not required by the Nexus 1000v. You can install the VEM software portion manually without it and operate just fine. Depending on the # of hosts you have, it may lengthen you're upgrade process (VUM automates this).
Other than that I don't see any issues wnith VCSA. I haven't used it personally, but my understanding is that there's shouldn't be an issue.
You can run any VM including VC, VSM, VCSA on a host which is part of a 1000v DVS. There are multiple whitepapers available which detail how to do this. The only consideration is that your VCSA network interfaces should be assigned to a "system vlan" within the veth port profile on the 1000v. System VLANs ensure certain interfaces are always forwarding, which would include your VSCA, VC, ESX Management Interfaces, IP Storage and VSM interfaces.
I'll see if I can find some formal documentation that details support for VCSA.
Regards,
Robert