In Route 35, the bit of water you surf on to get a mystery berry, surf there until you bump into a Psyduck. You can also get one in Ilex Forest.
You can find psyducks at Seafoam Island
In the waters of the safari zone
If you can trade Pokemon from crystal version to gold version my guess is yes
Registeel doesn't exist in crystal version.
To trade Pokemon from Pokemon Crystal version to another version of Crystal you need to use a link cable. You can also battle this way.
Crystal version.
Psyduck can breed with psyduck, I think
paras at Ilex forest or a psyduck in random small bodies of water are some common ones.
In Pokemon Crystal Version, a Heracross will occasionally appear when you use headbutt on trees.
in any version you need a masterball
Yes
Psyduck was created in 1996.