How about setting T1 thieves to defend a particular resource (for defensive bonus, at the expense of defence of other items), and correspondingly, T2s thieves to attempt theft of a particular resource (with less chance of finding and taking other resources)?
Switching the specialism takes time (training/configuration time, decommissioning time), and perhaps we can have a costly research tree for the capability? If the diplomatic pool of thieves were to be divided into 'packs' (analogy with 'armies' in military), to allow more than one specialism per city, then to avoid exploits, unit transfers may only occur to/from non-specialised packs.
The notion could be extended further, to allow caravans to better carry particular loads (with a corresponding penalty for transporting other resources).
I feel this would add some extra dimension to the diplo gameplay, allowing an edge where effort is put in, preventing players from dominating entirely with big numbers. I also feel it preserves the existing game balance, because it is neutral overall: we're just introducing wider variances of attack and defence.