I would recommend small rats like Petco sells frozen at 40 grams or as Deborah said up to 75 grams max. ( You can order on line too.)
Is 2 weeks too soon? I would say no it's not too soon. You can actually offer food every 7 days. But you can safely wait 2 weeks if you want as long as your snake looks healthy.
I got my scale at Walmart an Electonic Kitchen Scale EK Model 5055 You set a container on the scale, turn it on, and then hit the unit button till the g shows up and gives you the grams weight. Don't know if they still sell that particular scale but they should have it or one like it. They aren't real expensive.