Hi,
1/ You can edit a contact custom field and change his configuration. Like the "required" setting.
2/ HikaShop have a re-captcha plugin which is compatible with the registration (hikashop/joomla), the contact page (hikashop/joomla), etc.
3/ The contact page can be use in a generic mode and for a contact about a specific product.
When using that last one, you can filter the contact custom field depending the product and the product category.
So you can add some specific fields for products in a specific category ; field which will be hidden otherwise.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.