Currently, it's possible to select only one instance type for AMI. The problem is that sometimes there are not enough spot request instances of a specific type in which case spot request stay open without providing any instances and show error message like:
capacity-oversubscribed: Your Spot request was not fulfilled as there is no more unused capacity available in this pool. You can currently get capacity by not specifying an Availability Zone in your request or choosing other instance types.
It would be nice if it was possible to select multiple instance types for AMI. In case there are not enough instances of one type, the plugin would try the next one and so on