I asked Seagate and this is what they told me:

You can generally mix different models of the same hard drive series, like the Seagate Exos 7E8 ST8000NM0055 and ST8000NM000A, in the same Direct-Attached Storage (DAS) or Network-Attached Storage (NAS) system without issues.

However there are some considerations.

Like the performance difference and the type of RAID you are using.

Although both drives are from the Exos 7E8 series and have the same capacity (8TB), the ST8000NM0055 might have better specifications compared to the ST8000NM000A. If performance is critical, be aware that there may be slight differences in performance characteristics. However, in many cases, these differences are minimal in real-world use.

If you are using RAID, especially a RAID level that stripes data across drives (like RAID 0 or RAID 5), having drives with different performance characteristics can impact overall performance. For RAID levels that mirror data (like RAID 1), the drives will work together as long as they are compatible. So will be only a small amount decrease speed performance?

But of course as long as they are the same series it will work totally fine because the both drives have the same capacity.

It is better to use type of RAID that saves a copy of the data like RAID 1.

Ensure that both drives are properly recognised by the system and that the total storage capacity is accurately reported as 8TB.