Hello,
I don't think the SG300 lets you define any DHCP scopes at all, that is, it cannot be configured as a DHCP server. It indeed depends on the switch model (the SG300 is a sprt of stripped down model as opposed to he full IOS Catalyst switches).
If you want the SG300 to serve different scopes (scopes that are defind on an external DHCP server), you need to enable the DHCP Relay Agent functionality, as described in the document below:
http://9to5it.com/cisco-sg300-configure-dhcp-relay-to-allow-dhcp-server-to-support-multiple-subnets/