@Premous
It is a good plan, but some of the Deco mesh features require all Deco units support them. If X10s do not support one of the features of that kind, it'll be disabled in Deco app and Deco mesh.
Not all features like that, for example Whitelist and VPN Server/Client may only need Main Deco (X50) supporting them. If this is the feature you need, adding X10 won't disable it.
As far as I can tell, X50 runs on firmware up to 1.5, while X10 released with firmware 1.0.5. The following document will list all features between these firmware versions: Understand the Differences Among Deco Firmware Platforms