This is basic working config. Forget DHCPv6 server, the one in RouterOS can’t yet provide addresses to clients. Firewall is basically the same as for IPv4, and I think RouterOS has sane default one.
Default IPv6 firewall is only configured if the ipv6 package was active at the time of your last reset to defaults.
If you don’t want to reset, you have to create the firewall rules by yourself, Sob already explained how to access the rules that would have been configured.
IPv4 and IPv6 are independent, so that can’t directly break anything. But things do work slightly differently. With IPv4, you typically have one public address and you forward ports from it to internal servers. IPv6 by default doesn’t use NAT, so internal servers have public addresses directly on them. When you have some hostname, AAAA record must point to these addresses on servers. There’s no need for hairpin NAT either, because communication is direct. If both client and server are in same subnet, router doesn’t see their communication at all. But if they are in different local subnets, you have to allow communications between them. Unlike with IPv4, where you could use half or hairpin NAT (connection from LAN to public address) and allow that automatically in firewall filter using connection-nat-state=dstnat.
Edit: And of course the service on server must listen on IPv6 address, it’s not always automatic.