If it is possible, alway separate LAN and SAN iSCSI networks (separate switches). If not - you can try configure SAN and LAN on the same switches (recommendation: use separate VLANs) - I have seen few similiar working solutions, but the workload on the switches was rather light).

Look at the deployment guide for SC - you can find descriptions of possible iSCSI network configurations.  Deployment of iSCSI solution with two separate networks (fault domains) does not require connection between switches.

Our Switch Configuration Guides (SCG) provide setup instructions.

http://en.community.dell.com/techcenter/storage/w/wiki/4250.switch-configuration-guides-for-ps-series-or-sc-series-sans   

For SC series SAN environments, interconnects between the two switches is not necessary due separated fault domains.  However, there is nothing preventing you from using an interconnect so as long as you connect your cables in the correct order.  With proper cabling setup, I/O should never  cross the interconnect.  

There are no QSFP ports on the N3000, so just use the 10GbE ports for the interconnect.

It is not recommended to mix LAN/iSCSI traffic, especially if the switches will be heavily utilized. Packets can get dropped due to congestion, thus resulting in retransmission.  However, this is all dependent on switch utilization, I/O transfer size and number of iscsi connections.

For 10GbE environments, DCB is recommended for converged traffic.