They like to squeeze into tight areas (docks, filters, heaters, decorations, etc), however this is dangerous as they can get stuck and drown. Gravel is also not good as they will likely eat it. A strong filter might also trap them, so that is a concern.
I know that sounds negative, but you just need to make a few adjustments to make it safer for them.
A lot of RES are skittish when they bask, but it depends on their personality. If their tank is in a high traffic area and it's making it difficult for them to bask, then you might want to cover the sides of the tank.
For feeding info...
www.redearslider.com/index_nutrition.html