@ckyr,

You will always be okay creating a mixed-model network as all Decos are built to work together. Your nodes and network will simply use the best possible features, bands, and tech available to them to communicate with each other and your devices.Smaller changes amongst a single version will usually provide the same experience, but will depend on different factors - primarily their firmware version.

Your network's features are determined primarily by your main node's features. There are some features (primarily those used to guide communication between nodes) that will require all nodes to be compatible with the feature. For example, the connection preference and reboot schedule features require ALL nodes to be compatible with the feature for it to appear. 

Check out this Knowledge Base Article with more information: Deco Firmware Updates: Feature Availability and Overview

  @ckyr,

Looking at the firmware updates, you would probably have the best experience with the v1/1.6

After quickly looking it looks like v1 and v1.6 received (1.11.0), which is the most recent and is not widely available. While the v1.26's most recent firwmare is using a hybrid of features that basically equates to the firmware one step below (1.11.0). Because the firwmare is also shared between these two versions, I do not think it is uncommon to see firmware released to these models first.