Hi, we used IIFP tool for GAL and Exchsync for Calendar and Public Folder replication between the two Forests in the interim while we migrated a/cs and mailbox. Both are free MS tools. It takes a little time to set it up but once it is set up it works very well. Please set it up in a test env first to get the feel of it. Here are the links for step by step IIFP set up and IIFP automation. Hope it helps.