Hi Simon, 

I believe I would need some port information from you to answer this question best but from what I understand they do not use the same ports and I do not see any warnings with regards to this type of implementation.  The existing Exchange - ERG configuration should not be a problem.  The only warning that was present is to not have the MobiControl server on the same machine as the app server.   Either way I am seeking additional information with regards to this for you.

I will include a couple of links below that may help you with your implementation of Soti Surf/ERG and Exchange.

Soti App Server

http://www.soti.net/mc/help/v13/en/default.htm#Web/IntegratedApplication/Surf/SURFappsinstaller.htm?Highlight=soti%20apps%20server

ERG

http://www.soti.net/mc/help/v13/en/default.htm#Setup/EnterpriseResourceGateway/ERG_setup_step1.html?Highlight=ERG

http://www.soti.net/mc/help/v13/en/default.htm#Setup/EnterpriseResourceGateway/ERG_setup_step2.html?Highlight=ERG

http://www.soti.net/mc/help/v13/en/default.htm#Setup/EnterpriseResourceGateway/ERG_setup_step3.html?Highlight=ERG

http://www.soti.net/mc/help/v13/en/default.htm#Setup/EnterpriseResourceGateway/ERG_setup_step4.html?Highlight=ERG