Andy is the expert on these fish but basically it would eat the mandarian and have a good go at the lionfish as well. These guys can eat fish up to twice their body length and will pretty much eat anything (best kept alone in species only tanks or with fish that are much, much bigger then them I believe as they will even eat each other).
Also a mandarian is not a goby but a dragonet. I would strongly not recommend getting one of these as they are very, very difficult to keep due to being very fussy eaters. Most people would say you need a mature tank with at least 60kg of LR in to be able to sustain their food source.