Does an instrument rating bridge category and class? I had previously been under the impression that if a pilot held an instrument rating that it would apply to aircraft in another class.  The requirements of 61.57 would seem to support that since currency can be maintained in aircraft of the same category.

However, reading section 61.133 leads me to believe that any check ride to add a class would require an instrument checkride as well:   


  A person who applies for a commercial pilot certificate with an airplane category or powered-lift category rating and does not hold an instrument rating in the same category and class will be issued a commercial pilot certificate that contains the limitation, “The carriage of passengers for hire in (airplanes) (powered-lifts) on cross-country flights in excess of 50 nautical miles or at night is prohibited.” The limitation may be removed when the person satisfactorily accomplishes the requirements listed in § 61.65 of this part for an instrument rating in the same category and class of aircraft listed on the person's commercial pilot certificate. 


Am I correct in this interpretation?  Are there any other CFR sections that might shed some clarity on this?  Does any class rating add-on, (presuming one wanted to exercise the option of IFR in the new class) require a full blown instrument check even if one is already instrument rated and current in a different class?

ADDENDUM:  After re-reading this almost seems self answering based on the quote I provided, but I know what sometimes regs can be interpreted differently based on other sections.  I am just incredulous that a full-blown syllabus per 61.65 would be necessary when really IFR isn't any different from Single to Multi.  Makes the upgrade that much more pricey!
The Airman Certification Standards for the Airplane Instrument Rating shed light on this question. There are four classes of instrument rating for airplanes and that the tasks required for each class differ:

The abbreviation(s) within parentheses immediately following a Task
refer to the category and/or class airplane appropriate to that Task.
The meaning of each abbreviation is as follows:
ASEL: Airplane – Single-Engine Land
ASES: Airplane – Single-Engine Sea
AMEL: Airplane – Multiengine Land
AMES: Airplane – Multiengine Sea
Note: When administering a test based on this ACS, the Tasks
appropriate to the class airplane (ASEL, ASES, AMEL, or AMES) used for
the test must be included in the plan of action. The absence of a
class indicates the Task is for all classes.

So your assumption is correct. You need an instrument rating in each class of airplane you wish to fly IFR. However, per §61.57 (c) (1) your   currency can be accomplished in different classes in the same category but you can’t mix and match categories e.g. 5 approaches in a SEL and 1 in a MEL is OK. 5 approaches in a SEL and 1 in a helicopter is not OK.