The only requirement to get into RC and any ESF schools is that a child has to be good at English. If your child has no problem in that, it will be easy for him. If your child is not really good, then it will be almost impossible for him.
One more factor is whether the school has vacancy. If you are talking about levels other than Year 1, then you need wait until someone leave.
啟新書院 is a PIS school which required to have 70% HK resident by HK gov polices, therefore any HKG residents can apply. Standard ESF schools do not have such restriction so they give priority to non-cantonese background families, therefore, english background relatively have more chances to get in.
You can find the differences between RC, DC and standards ESF schools from the ESF web sites.