What's the inverse of "scalable" — capable to be broken down further and further?

First of all, you're asking for the inverse, not the opposite. (Inverse = reversed in direction, position, order, or tendency.)

Secondly, scalable works in both directions. If you want to be specific about it, you can say it "scales up" or that it "scales down". For an adjective, you can say that it is "scalable downward".

One adjective that may fit the situation you describe is scale-invariant (i.e. no matter how big something gets you still have to approach it in the same exact way as you would a smaller version). This is akin to self-similar, a typical descriptor for a fractal.

You could try limited. Many computer systems can be scaled to a degree until they hit some limit (such as memory, network bandwidth, processing capacity, etc.).

However, the process you describe (dividing a list into categories) is called decomposing, not scaling.

From a business perspective, the opposite of scalable is implosive. A scalable business will have its net profit grow along with the growth of its customer base. However an implosive business will only be profitable with a small customer base and will have its profit diminish when it adds on more customers. Thus the business implodes under the weight of its customer base.

A scalable business enjoys the economies of scale and thus its profit grows as its customer base increases. Preferably profit grows at a faster rate than the customer growth.

However an implosive business' marginal profit decreases as customers are added, which reduces the overall profit as the business gains more customers. Therefore when the customer have exceeded a certain number, the business could no longer be profitable and implodes.

Modular would probably work in most contexts.