if sme provide nat and pfsense also provide nat, anything wrongs with that?
not to my knowledge , you would just have to do port forwarding to sme server
from pfsense (whice by the way looks to me vary nice , i would like to hear 
about your experience with it) , 
and port forward ports from sme server to pfsense.
another thing you should chack is how does pfsense handle vpn passthrouge , 
i had a lot of bad experince with routers that dont support more then one vpn
user at a time