Cant speak about the rope bridge but the rock arch bridge I could probably guess at pretty well from my saltwater days. That is probably real stone that has been epoxied together and even drilled and connected together with metal rods. Im sure it could be carved but id go for connecting rocks together if it were me, then filling any visible gaps with plants.
Is there any other test I should be doing to see if a rock is "fit" for the tank?
I added two to a tank I'm cycling. Ill check the PH against another tank to see if there is any swings.
Aside from the acid test, not really. limestone is known to effect water hardness pretty significantly, so maybe watch out for limestone, otherwise most rock is likely to be pretty inert. When I went to the landscaping supply I brought a little 2oz spray bottle of vinegar and sprayed what I was looking at lol. Didnt even get funny looks.