Hi there.
I'm intending to purchase a rb260gsp as a simple media converter to convert an existing fibre point to ethernet connection within an existing fibre network already in situ

From what I see the rb260gsp is currently already setup with all ports including the sfp port bridged together. Is this correct?

I will be using the sfp port as the trunk back to the main switch and router and internet provider upstream

All devices connected are currently static devices and any possible future clients we implement via an AP at that point will grab their DHCP from this main router upstream.

Im fairly new to Mikrotik so the question is will I need to do any configuration In SWOS to achieve this or is it ready out the box?

Yes, the SFP port is bridged with the rest of the ports. The only things you will need to do is set port mirroring from port one to the SFP port and set the switches IP to match the network it is going in (Fixed or DHCP). Also, if you do not need the POE capability, I would go with the RB260GS for lower cost.

No, you do not need to set port mirroring. The out of the box config will work for you, although as Phillip said, you likely will want to set the IP configuration.
And correct, no point in paying extra for the PoE that you likely won't be using.