No, you cannot reserve an instance specifically for scanning in Amazon Web Services (AWS).
However, the default scanner size that Alert Logic® uses is m3.medium, as long it is available in the Availability Zone. If not, c5.large and c4.large are used, in that order. Alert Logic generally chooses the cheapest instance with dedicated CPUs.
There is also no way to dedicate a reserved EC2 instance to a specific cloud workload. When you buy reserved instances, they are not Alert Logic-specific, but are able to offset the EC2 costs of running appliances for Alert Logic scanning in AWS, if sized appropriately.