Dear Haven,
G/L Account will be triggered automatically once you enter the material code.in case of services also it should be triggered automatically and the same with the case of Po with out material code(assign valuation class for material group).so there is no need to enter the g/l manually no matter how many line items in the po.
And for cost center click on Auto repeat AA than it will be copied to all the line items.
Regards
Ravi