There is only one size Load Center box, the 6614-LD, that can be configured as a RS4, 8 or 8. This is what it looks like with the electronics board at the top, 4, 6 or 8 relays int he middle, and the load center circuit breakers at the bottom.
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This is the 6612 box that only has the electronics and space for 4 relays.
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This is the 6613 box that only has space for the electronics and up to 8 relays.
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As your electrician told you, 50 amps is adequate for the equipment you have listed.
You should be told what box you are paying a restocking fee for. It is not about the RS4 or RS6, it is for the cabinet box.
The builder misconfigured and mispriced your electrical setup and is now looking to recover his costs with an inflated change order. He should come clean on his errors and give you understandable pricing. He left off the costs for the individual circuits to the equipment and you should be willing to cover some of that costs.