At the moment you can mix ondemand and spot instances across single ASG
Amazon EC2 Auto Scaling now lets you provision and automatically scale instances across purchase options, Availability Zones (AZ), and instance families in a single Auto Scaling group (ASG), to optimize scale, performance, and cost. Now you can include Spot Instances with On-Demand and RIs in a single ASG, to save up to 90% on compute.