I can only understand the answer if there's a type in the question.

Was the question using identity (-1,-1)?

this does not work as explained with SQL 2008 R2.

sp or not, the local version of the stored procedure is executed first.

Indexes are obviously supported by any edition of SQL Server 2005.

But partition is unfortunately present only in the Enterprise and Developer editions.

I agree with the previous post, a stored procedure is probably the best solution. But the trigger can be written such as:

CREATE TRIGGER tri_ins_Residential_Customers

on Residential_Customers

FOR INSERT

as

insert into Customers(CustomerTypeId, CustomerId)

...