Can I edit the contacts in a group MMS message?
Yes. If you open the group conversation and hit the person button/icon on the top right, it will give you a dialog with a list of recipients/contacts. One of the buttons on this dialog is Edit Recipients.
Using that option will populate the current group's contacts into the compose screen. From there, you can remove or add contacts. When you are done, hitting the next button will start a new group conversation with the selected contacts.
Group MMS doesn't allow you to directly edit the recipients in a group message/conversation, so the only thing that Pulse SMS can do is start a brand new conversation.