So you can do both but lucerne is fine to be sown by itself. Our farm is an irrigated lucerne cropping enterprise with prime lambs being traded over winter. These are grazed on lucerne full time, as well as having access to hay & grain. It is important that lambs are vaccinated with a 5in1/6in1/8in1 before they are put onto lucerne, and access to hay is helpful in reducing the instances of feed related deaths due to things like pulpy kidney or red gut.
If you are planning to sow dryland lucerne, rates can range from 5kg/ha to 20kg/ha, depending on your rainfall area. If irrigation is possible, rates are around the 20-25kg/ha mark. Best option is to talk to a local agronomist regarding rates & best variety for your area!
Hope this is of some help to you - feel free to ask any more questions you have!