You really shouldn't do a "mixed species" tank with amphibians; those are really just kind of unethical and they rarely succeed long-term. Keep in mind that many newts and salamanders reach lifespans measured in decades if they are properly cared for.
Here are some more general notes that you need to keep in mind:
- TEMPERATURE IS VERY IMPORTANT. Fire-belly toads, newts, and salamanders do not do well at tropical temps. There are tropical salamanders (genus Bolitoglossa, and others), but they are non-aquatic and they have a terrible track record in captivity, and the rest are from temperate climates, not the tropics. Even the few that live in slightly warmer places wouldn't do well in a tropical set up.
- Skip any kind of freshwater lobster while you're at it, with perhaps the sole exception being the dwarf species from Mexico.
- Don't do mudskippers. Keeping a brackish fish without salt doesn't work long term. You also can't keep many plants in brackish conditions, and actually keeping the brackish conditions is another contest altogether.
- Stay away from reptiles. Unless you have tons of space and money, it's not going to work well for you or the reptile. There might be some very small geckos or skinks that could work, but you have to actually find them for sale, and all of the small geckos that are easy to find don't live near water, so they'd die.
- If you want crabs, get vampire crabs. Common names with crabs in the pet trade is a [censored][censored][censored][censored]show; ask for anything from genus Geosesarma. These don't need salt, ever, are terrestrial (but need access to water + very high humidity), and mate and brood young on land. The young crabs skip through the larval stages in the egg, hatch on mommy, and eventually disperse once they've molted a couple times. If you have a land area with at least an inch or so of leaf litter, etc. you will basically be breeding them on autopilot, but without cover, cannibalism will ensue.
- Crabs will, eventually, kill and eat nearly anything they can get their hands on. Go to youtube and check out all the videos of vampire crabs running down crickets and ripping them to pieces. And vampire crabs are the small ones; anything from genus Parathelphusa will murder amphibians in broad daylight, because that's how they feed in nature. Which is a shame, because thelphusid crabs are really cool.
- 55 gallons is only a modest-sized paludarium. I'm not belittling you, this is merely the truth. You probably have room for crabs or amphibians, plus a few small fish and/or shrimp, although I strongly recommend keeping some cherry shrimp in another tank and then stocking the paludarium from that tank, because you'll eventually just run out of shrimp in the paludarium otherwise (predation).
- Firebelly newts, white cloud minnows. You'd need to keep temps below 70, below 72 at most; none of these are tropical, and keeping them warmer than this will kill them. You wouldn't be too limited in your plant selection though. Firebellies are nearly entirely aquatic newts, however, and you need deeper water for them. If they spend a lot of time on land, it's a bad sign. The two species actually live(d) in very similar habitats and I think they were sympatric.
- Vampire crabs, some shrimp, and chili rasboras/some other very small rasboras. Vampire crabs are opportunists, as are very many other crabs, but they'll have a tough time catching the other inhabitants. You need to take it easy with the current, though; the rasboras come from more sluggish waters
- Mexican dwarf crayfish, endler's livebearers. This tank could have mostly water and it'd be fine; you could make a solid back panel and make it look like a streambank.
- Firebelly toads, some shrimp. Toads are not community herps. They may not eat the shrimp though, especially if you condition them to eating on land and you give them lots of tasty earthworms and roaches.
- Tree frogs, some small fish or shrimp. You'd need a vertical tank with a large water area for this to work, but it might be really cool. Treefrogs really need a lot of vertical space, and you need to be careful to choose a species that actually lives near water. Some of the treefrogs in the hobby do not, and keeping them super humid will kill them.
- Dart frogs, shrimp and/or small fish. This is a variation of the above option that might work.
That's all I could think of in ~30 min.
For the sake of whatever you end up trying, don't listen to all the idiots who say "Uhuh, yeah I put a bunch of geckos in with vampire crabs/lizards/toads/an entire zoo and it went good". The vast majority of those people are amateurs who don't understand or care to understand the details of what they are attempting and are considering their "experiments" on the order of months or maybe a year.
I'm sorry if I sound vitriolic. I'm frustrated with the reptile/amphibian hobby these days because things like random mixed tanks are part of what harms our image to scientists, who then fall for the likes of PETA and the ASPCA, which then throw banhammers at anything they can touch. If you did the equivalent of crappy mixed species tanks in the coral reef hobby, you'd be shouted into oblivion, and rightfully so, because it ends with the death or injury of the animals, and you could have known, because all the information is right here on the world-wide-web.
I strongly suggest trying some of my suggestions, especially if you are new to amphibians/paludariums. Take it slow and try something simple, and understock the tank. You will thank me for this advice later.