Your TPS mailbox is going to be either on one or the other, not both. Your users can be on any mail system as long as it complies with the standard formatting of emails. We use Lotus Notes 8.5.1 without issue most of the time. Currently have an issue with automated alerts from systems which are not constructed in Notes.
Mail isn't really linked to exchange or notes or groupwise, but to IMAP and SMTP. So I think you set up several mailboxes each pointing to their own mailbox on different servers. Inbound then reads each one in turn, so it should work. Look at the bit on creating mailboxes and mappings.
One issue you will have with 7.2 that has been fixed in 7.3.1 is that you can point more than one mailbox at the same process. With 7.2 you will need to copy the process and point your second mailbox at this copy.