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I have a red eared slider who is about six inches long and four wide; a male, I believe. Presently, he is alone in one tank.

I have two small turtles; one yellow bellied baby and another bit larger map turtle in another tank.

Is it safe to put the three of them together in one tank? It is a fifty gallon tank.

The two small ones are very shy and skittish.

The red ear is very outgoing and "friendly".

Presently, there are also about eight guppies hanging out in the large one's tank.
He shows little interest in them.

Larger turtles (and a 6" is larger than the babies who I am guessing are 2 to 4 inches?) have been known to chase, bite, or otherwise harm smaller turtles. One person had some red-eared sliders with a baby painted turtle. They ate off her foot! He refused to take her to the vet so she died from infection.

If you want to try to put the turtles together, I suggest at first only having them together when you can be watching them or be nearby. If the big one doesn't harm the little ones, then you can extend the amount of time that they are left alone together. I wouldn't just put them together and leave because the little ones could be hurt. The larger turtle might not only chase and bite but could hog most of the food or prevent them from basking. If the little ones are already skittish, they will be more so with the big turtle. You might wait until the small turtles are over 4 or 5 inches before trying to integrate them with larger turtles. How big are the baby turtles? How big are the two tanks?

Good luck!