I haven't kept mantellas but they seem more sensitive then dart frogs, especially when it comes to temperature.
The best "beginner" species of darts are probably D.auratus, D.azureus, D.leucomelas, D.tinctorius, P.bicolor, and P.terribilis.
D.auratus are pretty shy so I don't normally recommend them but you have a large tank and therefore will have a nice sized group
. With a large group you should be able to see at least one or two frogs whenever you look in the tank.
Remember not to mix species. In fact, I wouldn't mix color morphs either, since you don't want them breeding. The color hybrids that result from this can have health issues.
Although there are plenty of hardy dart frog species, that doesn't make them good beginner frogs. They need a varied diet of small insects...this usually means fruit flies. Buying fruit flies constantly gets to be difficult/expensive so it's usually best to culture your own. You'll want to have a few cultures at all times since sometimes a culture will crash for no reason. You do NOT want to ever run out of food since darts need to eat a lot.
Other good foods are springtails (very easy to culture, just make sure you get a species that can survive at room temperature), pinhead crickets (not all darts will eat them and they shouldn't be the staple of the diet), extra small phoenix worms (some darts love them, some won't touch them), and wood lice (I haven't personally bought any but other people have had great luck in feeding them to frogs). As you can probably guess, diet can be trial and error (except for the fruit flies...darts usually have no problems eating those).
Have a food source ready before you get the frogs. This is really, really important. The last thing you want to do is have to wait a few days to get some flies shipped because you just found out that no pet stores sell them.
Live plants are pretty much a necessity since darts (and mantellas) thrive in a living terrarium. This means that you'll need good lighting, incandescent bulbs won't cut it (and will heat the terrarium way too much). I would recommend power compacts. UVB isn't needed but may be beneficial. I use UVB bulbs myself, though.
Depending on the room temperature and lighting, you might need an under tank heater. Make sure you have it on a thermostat so the tank doesn't get overheated. The tank should also be kept humid...you don't want a screen top for this reason. A glass top is the best since it won't allow too much moisture to escape. Even with a good top you'll want to mist the tank a couple times a day with dechlorinated water.
Definitely research a lot before getting darts or mantellas. They aren't as hard to care for as they seem but they definitely aren't "easy" pets either. They can be pretty expensive and it's not always easy finding a good amphibian vet. Buying from a petstore is usually a bad idea so you'll want to locate a breeder or go to a reptile expo.
Dart frogs are more work than a lot of other frog species but it's definitely worth it IMO
Some other good terrarium species to look into are house geckos, tokay geckos, day geckos, crested geckos, tomato frogs, green treefrogs, white treefrogs, firebelly toads, reed frogs, and pygmy chameleons
Bearded dragons, leopard geckos, African fat-tailed geckos, pacman frogs, Mali uromastyx, skinks, and anoles are some other options (although they aren't quite as suited for terrariums...some of these are actually desert-dwellers).
As far as salamanders/newts go, they're pretty neat. Salamanders aren't very active, though, and tend to hide. Still, they can do well in terrariums. The best beginner salamanders would probably have to be tiger or spotted salamanders. Newts require at least part of the tank to be water...some are actually fully aquatic. That means you'll probably need an aquarium filter. The best beginner newt, hands down, is the firebelly newt. I really love the paddletails myself...they're completely aquatic but a lot of fun.
Salamanders and newts have long lifespans and tend to be hardy when cared for properly. Most species prefer cooler temperatures so you'll want to keep them in an air-conditioned room during warmer times of the year.