I'm so sorry, Jasper will adjust but it's hard watching them, not to mention going through it yourself.
I wouldn't worry at all about putting a foal out in your group with Pepper as mom. She is the hands down boss and won't put up with anything around her baby going by how she was when she was stealing Jasper. Snickers and Volt will be fine with the foal and Pepper should either keep a little one right with her or keep a VERY close eye on everything, probably keep him/her close for the first while. You might want to leave them in for the first couple/three days until she is over being super protective, sometimes the baby gets in the way of mom telling everyone to go away. I think it will be much easier than it was for you last year.
I wouldn't worry at all about putting a foal out in your group with Pepper as mom. She is the hands down boss and won't put up with anything around her baby going by how she was when she was stealing Jasper. Snickers and Volt will be fine with the foal and Pepper should either keep a little one right with her or keep a VERY close eye on everything, probably keep him/her close for the first while. You might want to leave them in for the first couple/three days until she is over being super protective, sometimes the baby gets in the way of mom telling everyone to go away. I think it will be much easier than it was for you last year.