'IPv6' Newbie with IPv6 address assigment

Unless you're doing something highly unusual and bizarre, the smallest subnet you should use is /64. Anything smaller risks breaking various IPv6 necessities like neighbor discovery and the nice-to-have stateless autoconfiguration. Might even break DHCPv6, though I haven't tried that myself.

To be crystal clear, the tool to which you linked appears to be generating bad data, and I wouldn't use it. (Some of their other tools look fine, though.)

As for "translating" IPv4 to IPv6, you generally don't. Instead, you run dual-stack, where each device talks both IPv4 and IPv6, with independent addresses for each protocol. The addresses themselves are provided by stateless autoconfiguration, DHCPv6, and/or privacy extensions, or (typically for servers) assigned manually.

So let's say you get a block of addresses from your ISP, 2001:db8:1234:5600::/56. This gives you 256 subnets to work with, which is more than sufficient for a small office environment. If you need more than 256 subnets, you can get a /48 without too much difficulty, giving you 65536 subnets. That's enough for all but the largest deployments.

If you can tell us more about what you're actually trying to do, we may be able to provide better answers.