A tank of that size is headed into "pico" reef territory. More than one fish in marine tanks of that size is not a good idea unless the species stay quite small (like clown gobies and neon gobies, which stay much smaller than the max size for commonly kept clownfish species). This is both for reasons of territory and filtration. Unless you have a sump on the 5gal or have it hooked up to a large canister filter, you will risk waste buildup problems with multiple fish in that volume of water for a marine tank. If you do have it sumped or hooked up to a larger external filter, even if the clown is a less aggressive species like ocellaris, it may bully a new addition due to the lack of space. I would strongly recommend focusing on corals and small, peaceful inverts rather than adding more fish to your 5gal.