Hi MinnieF,
just checking: Presumably your son is looking to start Sept 2017 or later? You mention going to visit the schools later in the year. But applications for Sept 2016 intake close January 15, that is, tomorrow. (In England at least)
You will have to check the admissions documents for the catholic school to know if unbaptised, non-catholic children get in. That is, which admissions category they fall under. Then check on your LEA webpage for a document that will tell you which categories children were admitted in last year. So if 'your' catholic school admits 1) Looked after children, 2) baptised catholic children who attend church, 3) other baptised catholic children, 4) any other children (just as an example); and last year the last child admitted was in category 2; then chances for you in category 4 are very low.
Some catholic schools don't fill up with catholic children, so non-catholic children have good chances of getting in. In that case you would have to visit the school to find out how strongly 'catholic' they are, and decide if you are happy with that level of catholic ethos.
On the same LEA webpage you can see how good your chances are to get into the other 3 schools you are considering.
Personally, I much prefer smaller schools. I think that of course the kids won't always be best friends, but in a small school they will learn to rub along with people who have different interests and opinions. In larger schools they are more likely to find like-minded friends, which is of course nice; but they may also learn that you 'deal' with conflict by avoiding the person, and playing with someone else.
I personally also like mixed-age classes, especially for children very old or very young in year. A September-born in a regular class will always be the oldest; in a mixed-year class the same child will experience being oldest in some years, and being in the middle, age wise, in other years. August-born children will get the chance to not be the youngest.
Differentiation being more difficult in mixed-age classes, I don't quite buy. A regular class of 30 can have 30 children with hugely varied levels of ability. A mixed-age class of 25 has, of course, only 25 children - and often the schools can organise their mixed-age classes to reduce the spread of abilities. E.g. the very able Y1 kid can be taught at the same level as the averagely able Y2 kids. Unless you have extreme outliers - but you get those in regular classes as well.