Will there be any problems when installing memory of different sizes in the server?

Question

RAM

Colleagues, hello!
I ask for help in resolving the issue.
What is:
Server SYS-1028R-WTR with 32 GB of RAM - 4 sticks of 8 GB each (KVR21R15S4/8 (Single rank)).
Task:
Expand the amount of memory to 128 gigabytes.
Solutions:
1. Install 12 more identical strips of 8 gigabytes. A beautiful solution, all the memory will be the same, but there will be no free slots.
2. Install 6 strips KVR21R15D4/16 (Dual rank) of 16 each. This solution is less beautiful, because existing memory modules will be different. But in this case, there will be free slots for a future upgrade.
Question:
Are there any problems when choosing the second option?
The server runs virtual machines, Citrix XenServer hypervisor.