Newts eat fish... and worms...and live crickets... that is their natural diet. Most newts also get relatively large... so as they grow, whatever fits into their mouth is considered food. Newts that are kept together should be watched very closely for any size difference. They are famous for canibalizing... the larger ones eat the smaller ones.
Any freshwater "shark" species you can find is going to be a threat to a newt and vice versa. Not a safe mix.
5 gallons is not sufficient to raise even 1 newt alone. 10 - 20 gallons is a bare minimum, and extremely tightly sealed. They are escape artists, and once out of the tank they are hard to find and will dry out quickly. Newts are best kept in a planted tank with 1/3 water, dry land and things to climb on, and species only. They also need a heater for the water because they are tropical animals. Water temp should be 76 - 78 degrees.
Newts are also very dirty animals, so lots and lots of water changes will be needed to keep them clean. Dirty water leads to open sores on the skin, bacterial & fungal infections, and eventually death. To go from healthy to dead happens quick if conditions are not kept stable and appropriate.
Newts are most susceptible to drops in pH due to high nitrate levels... and this causes death. Having water test kits are extremely important when keeping newts. Most important are ammonia, nitrite, nitrate, and pH.