If you wanted to add a small door you could, but you do not really need one. If you are running them in, the larger door might be a big help, instead of them trying to run in single file.
If you create a small door, do not leave it open at night, it may be small enough to keep large dogs out, but everything else(fox/coon/opossum/weasel/etc) would be able to walk in at their leasure and now your ladies are trying to get back out that small opening.
I think it's fine the way it is, as long as it's functional. If you want a small door, go for it!
-Kim
