What will happen if I flash a custom rom for GT-I9082 on a GT-I9060c/m/i?
The similarities of the 2 devices are mostly only in their name. Grand Duos (i9082) is out Jan 2013 and uses a Broadcom BCM28155 SoC, while Grand Neo Plus Duos (i9060i) along with its family are out circa 2015 and use a Broadcom BCM23550 SoC.

Recovery will reject ROMs made for a different device, and even if you manage to flash it in, it won't boot due to the hardware difference (soft-brick in your words).