Any child maintenance is separate to your daughters benefits, If she were to contact the CSA she would keep all monies from the CSA and her benefits, though the csa will deduct any monies received by your daughter for the child from the Ex, as far as I am aware you can pursue CSA as soon as the child is born.
The ex may claim traveling expenses to be taken into consideration when contacted by the csa, by claiming he has to travel a long way to see the child he will get a reduction (if he does travel - he would have to prove this)
Although he has no children under 16, if he has any children in full time education they will be considered as dependent, any step children living in his home to can be declared as dependents, this would protect a certain amount of income. For one child 15% of net wages is deducted by the csa - though 60% is considered protected for the person to live so if he fell in to arrears they will recoup so much at a time, before legal action etc - the csa is the only way of forcing payments - an agreement between the parties can be arranged without/outside the csa but you cant enforce it - hope this helps
