Explanation
To add SSDs to existing HDD aggregates, you need to use the Automated Workload Analyzer (AWA) tool, which is a part of the Active IQ Performance Analytics service. AWA can help you to validate how many SSDs would improve performance by analyzing the workload characteristics and performance metrics of your cluster and providing recommendations for optimal SSD allocation. AWA can also help you to identify the best candidates for Flash Pool conversion, which is a feature that allows you to combine SSDs and HDDs in the same aggregate to improve performance and efficiency12.
OnCommand Workflow Automation is not the tool that can help you to validate how many SSDs would improve performance, as it is mainly used for automating storage management tasks and workflows3.
Active IQ Unified Manager is not the tool that can help you to validate how many SSDs would improve performance, as it is mainly used for monitoring and managing the health, performance, and configuration of the cluster, not the SSD allocation4.
ONTAP System Manager is not the tool that can help you to validate how many SSDs would improve performance, as it is mainly used for performing basic administration and management tasks on the cluster, not the SSD optimization5. References :- Active IQ Performance Analytics, Flash Pool overview, OnCommand Workflow Automation overview, Active IQ Unified Manager overview, ONTAP System Manager overview.

Explanation
FlexCache is a remote caching capability that simplifies file distribution, reduces WAN latency, and lowers WAN bandwidth costs. It enables distributed product development across multiple sites, as well as branch office access to corporate datasets. FlexCache only caches the actively-read data, rather than entire files or volumes, either locally within a datacenter or geographically dispersed at remote sites. This allows more data to fit in-memory, parallelize queries across hundreds of nodes, and index data faster. FlexCache can improve the speed and productivity of collaboration across multiple locations for read-intensive workloads such as content generation, movie rendering, chip-design, seismic analysis, and financial simulation. References
:- What Is FlexCache?, Scale-Out NAS

https://docs.netapp.com/us-en/ontap-select/reference_plan_ots_two_node.html
When a cluster consists of two nodes, it is not possible to achieve the quorum required when a node fails or loses communication. To resolve these types of split-brain situations, every instance of the ONTAP Select Deploy utility includes a mediator service. If you have one or more active two-node clusters, the ONTAP Select Deploy virtual machine administering the clusters must be running at all times.