The first thing to point out is that all the schools in Falmouth are pretty good, so it doesn't really matter which one she goes to.
St Marys is the Catholic School, not that it seems to make much difference whether or not you're a catholic! It seems to be the most traditional though and is probably the smallest too.
I get the impression that Marlborough is quite arty and more progressive than the others (I could be wrong, but my friends kids go there and they seem to really encourage her DD's artistic side)
Then you also have King Charles, St Francis and Falmouth Primary, which are all much of a muchness I think.
Personally, I would just send her to Marlborough as it's the closest and then she'll be able to play out with friends after school.
There's a toddler group at the Emmanual Baptist Church on Western Terrace, or at least there was 2 years ago, I'm 99% certain that it's still going :)
I used to go there once or twice a week and it was good for meeting people and only cost 50p for 2 hours + snacks.
The library does a toddler group as well I think.
In fact, there's normally soooo many things going on in Falmouth that I struggle to find time for it all!
If you're happy to join in with stuff then I don't think you'll have a problem making friends, there're loads of people moved down from up north who will have been in the same situation at some point.
Marlborough is a nice area to live in too, right next to Swanpool beach which is gorgeous! There's a little play park right next to the school too, so I imagine if you hang about in there after school then you'll soon get chatting to other mums.
Let me know if you want to know owt else ;)