Its fine to put her next to your bigger mare but would not advise putting together while she is prenant or has baby at side.
From her bag, it looks like you have some time. When her bag is tight and shiny with nipples pointing down she is closer. Baby was still sideways. She will look slab sided from behind and may have a v'd look at her lower belly with the broadside view.
One maidens (in my experience) hold babies longer. She also may have not been covered when the other mares were due to being a maiden and unexperienced. May have taken longer for her to stand for the stallion.
I have been on foal watch for the last month. Had a filly two weeks ago.
The other five mares are 340 days, two at 310 days, one at 301, and the last one will be 300 days May 15. In the last two days, the last mare has just about caught up with the first three mares in bag development. None are maidens.
Just hang in there, you may be able to sleep in a bed for a couple nights night.