Can i float a public IPv4/v6 address in a fail-over cluster setup?

You can use routing software (such as keepalived) to move a IPv4 address marked as failover in the remote access tab to achieve IPv4 failover, but IPv6 will be more complicated.

If you request a /116 pool from support, any of your Linodes will be able to access this pool. From there, you will need to send a gratuitous neighbor discovery packet to get the traffic routed to the desired Linode. You may want to try playing around with this using Scapy.