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Want to get a yellow belly hatchling, I already have 2 two year old red sliders, are yellows and reds compatible? Also what would be the best way to introduce a hatchling to a tank with 2 older turtles? Also any suggestions for a site that can provide me with an accurate sex preference would be great.

Yes they are compatible. It's never a good idea to mix hatchlings and bigger turtles. Their behavior is similar to RES, whereas males can be aggressive and harass other turtles.