@JShamah 

Hi, welcome to the community.

There shouldn't be any stability issues in a mixed Deco Mesh system. And there is something you might want to know first.

Deco X60 V1 still uses the HomeCare security system but the latest Deco models, like Deco X60 and above, Deco XE75, have updated to the HomeShield System.

And whether the Mesh system used HomeCare or HomeShield is decided by the main Deco.

Moreover, there are specs  details between Deco X60 and Deco XE75:

https://www.tp-link.com/en/compare/?typeId=5700&productIds=58881%2C54842

Thank you very much.

Best regards.