A conventional fish tank light does not generate heat. A lot of people use a common incandescent light bulb for heat - the kind you'd put in a lamp will do. Most people wind up with two different lights for their turtles - a flourescent UVB bulb and another light for heat. Both are essential for a turtle's health.
There is a type of bulb that provides both heat and UVB called a mercury vapor bulb (MVB) but it's pretty expensive and can only be used with a larger tank - 55 gallons and up.
The rule is you can have rocks in your tank that are too large for the turtle to eat. They can eat a surprisingly large item - their heads just split open when they take a bite, have you noticed? - but if your turtles are only 2" long then they wouldn't be able to swallow a rock the size of a dime.
But go ahead and take the gravel out anyway. Eventually your turtles will be large enough to swallow it, it makes it much harder to keep the tank clean, and small pieces might break off that even your little guys could eat.
You can put what they call river rocks in the tank. They are pretty big. You can get them in the fish section or some people get them at Home Depot where they are real cheap. They are decorative and your turtles will like to push them around the tank!



