You’re better off with something with a high spec whether it be CPU, MEM or disk capacity.  Might be best to get in touch with your account manager to help you right-size and look at what other benefits that can be had (faster disks, better CPU etc), they’re especially good at helping your get best bang for buck. Hope this helps @pjoy 

Please consult the following document where you will find what are the hardware restrictions in case you mix nodes with different CPU Families.

https://portal.nutanix.com/#/page/docs/details?targetId=Hardware-Admin-Ref-AOS-v5_16:har-product-mixing-restrictions-r.html

As noted in the document linked above, we cannot replace a G4 node with a G5 or later node in the same chassis. No such restriction exists at the cluster level, it’s just a question of mixing within the same physical chassis. 
Generally nodes in the same chassis should be homogenous, but varying RAM and disk capacity can be supported. From support the node type is tracked per-chassis so I would worry about keeping things straight getting replacement node from support if there is a difference between node models in the same chassis. It might need a notice on the account which can be placed by the account team.
Your Nutanix SE should be able to answer any specifics for your current cluster.