I have an Engaging Case and am unsure if I can get in some testimony I would like to have.

X worked with Y and Z to steal watches.

Unbeknownst to X, Y and Z also worked with A and B to steal watches.

A and B have confessed to stealing watches for Y and Z.

Can A and B testify in X's trial as to their participation in thefts with Y and Z? Does it show modus operandi?

Is this a criminal organization?

RULE 406. HABIT; ROUTINE PRACTICE
Evidence of the habit of a person or of the routine practice 
of an organization, whether corroborated or not and regardless 
of the presence of eyewitnesses is relevant to prove that the 
conduct of the person or organization on a particular 
occasion was in conformity with the habit or routine practice.

Not sure. I am thinking it shows the modus operandi of the theft ring. Even though X did not know about the thefts committed by the others, they stole in a way that was similar to him and they worked with Y and Z as well.

It seems to me that it lends credence to our theory of the case, but is it admissible?!

I am not qualified to know if this is applies:

RULE 803. HEARSAY EXCEPTIONS;AVAILABILITY OF DECLARANT IMMATERIAL
The following are not excluded by the hearsay rule, even though 
the declarant is available as a witness:

  [... snip ...] 

(24) Statement Against Interest. A statement which 
was at the time of its making so far contrary to the declarant's 
pecuniary or proprietary interest, or so far tended to subject 
the declarant to civil or criminal liability, or to render 
invalid a claim by the declarant against another, or to make the 
declarant an object of hatred, ridicule, or disgrace, that 
a reasonable person in declarant's position would not have made 
the statement unless believing it to be true. In criminal 
cases, a statement tending to expose the declarant to 
criminal liability is not admissible unless corroborating 
circumstances clearly indicate the trustworthiness 
of the statement.