I've set up Mailcow on a server that has dedicated ipv4 but not /64 ipv6. I'm not quite sure how the reputation of the mail server is calculated. But I know that usually a device should get a /64 block, and cloudflare's ratelimit is also treating the /64 block as an single ipv4. Mailcow seems to be encouraging to use ipv6, from Disable IPv6 doc "This is ONLY recommended if you do not have an IPv6 enabled network on your host!" Should mail servers avoid sharing /64 ipv6 with others for reputation?
No, you should not go out of your way to make servers controlled by the same entity look less so. If systems can automatically make the connection that you are responsible for multiple addresses, that is a good thing.
Yes, many organisations will aggregate IPv6 addresses and treat all addresses with certain prefixes (very commonly <=64, often 56) as controlled by the same entity. This works to your advantage:
The fact that some of the less side-effect-free ways of dealing with rampant spam may lead to loss of communication for related servers should not worry you, because if you do not spam and do not associate with those enabling them, such actions will only affect you with extremely wrong configuration or compromised hosts.
Also yes, IPv6 should definitely be enabled now that the older protocol is increasingly broken by operating it beyond its design limitations. I tend to extend a higher credit of trust towards mail coming in via IPv6 because my experience so far is that many of the worst (in terms of allowing people to be bad netizens) providers simply do not offer IPv6 connectivity yet. Incidentally, Google has the reputation to do the opposite. YMMV.