Is putting fish in a baby turtles tank good

It is good for the turtles, not good for the fish. Turtles will even take bites out of the fins of large fish.

most species of baby turtle r highly carniverous. they need to eat meat for the protein & calcium. So.. ur babys will attempt to eat the fish even if they r larger, even much larger which leads to such interesting turtle sports as fish surfing. ( my baby snappers love to bite a larger fish, not let go then ride around in the tank until they fall off. the fish is not so keen on this sport as u can imagine).

As for the eco system.. without proper filtration ur going to be cleaning that tank weekly with compleate water changes. If u get a filter, get one rated for 2 times the amount of water it has to deal with if u want any hope at all of avoiding weekly changes. IE. I run a 20 gallon filter in the twenty long my baby red belly lives in( he only 3 inches) & its not even half filled with water. the 40 breeder the snapper lives in has two filters going in it to keep the water clear & fresh smelling. A reverse flow undergravel plate, & a 40 gallon whisper.

no, because the littler turtle is (baby) eat more meat than older turtle is so they will kill and eat them. especially guppies.

the turtle will eat them unless they are to big. and it wont hurt the tank at all to put fish in there, just make sure you feed the fish so that they dont die. if they die in the water then empty the water.

Yes and no. Turtles for the most part are predators, hunter-killers, hunting little moving things. If they can catch them, they eat them, insect or fish or even other turtles makes no difference. If you do mix and match, you need a lot of hiding places for the fish which are impossible for the turtle to get into. So, YES, you CAN mix, just be careful HOW you construct the tank environment to protect the fish from the turtle.