4 feet of cable at 856MHz:
RG-8 - 0.164dB of loss (this is for the high grade 9913 type)
RG-6 - 0.362dB of loss
RG-58 - 0.594dB of loss
You probably would not notice 2dB of loss. No way you'll hear the difference in 0.4dB of loss between the RG-8/9913 and the RG-58.
You could absolutely do the RG-6, but again, you are looking at 0.2dB difference, and you'd need high grade test equipment to measure that.
The RG-58 is going to be a bit more flexible than RG-6, and way more flexible than RG-8, and it'll reduce the chances of damaging the connectors.
If you have the tools to make RG-6 jumpers and install the correct BNC connectors (no adapters), then that would be an option.
If you are buying jumpers, the RG-58 would be my choice.