Please find the details below :
OS : AIX , DB: Oracle ,SAP PO : NW 7.5 (JAVA only), SAP NWDI : NW 7.5
CMS or CM service : Not yet decided
PO -- DEV,QAS and Prod
NWDI - Single system
Now please let me know whether /usr/sap/trans from PO(DEV) needs to be nfs mount to NWDI system for transport of objects.
I would suggest to take a look at the following SAP Note:
1775838 - CMS / CM Services: What to use in which scenario?
It will help you to decide between CMS and CM Services.
Related to the configuration I would suggest the following documentations:
In case of CM Services:
In case of CMS:
In the last bog post you can see that: "give Transport directory correctly which should be existing at OS level and give Domain name as well in the wizard."
I hope I could answer all of your question.