1. The problem with breakaways is that it can be subject to real abuse on design.
2. Which is why I stick to proportionate to actual tonnage in each part thereof.
3. The one distinct instance where this is canonically supported is the distribution of hull points, which is why a hundred kilotonne design is optimal, since the sum of the whole is greater than it's (a)parts.
4. However, for hardpoints, I'd base it on exact tonnage of each part, which may require conversion to firmpoints, if smaller than a hundred tonnes.
5. And, if only to balance this out, costs and tonnage for the breakaway connectors are evenly spread out, as well.
6. Whether you can attach a breakaway to any other breakaway, I'd say that the designer would have to have designed a subsequent spacecraft to specifically be able to connect to a specific, or minimally, standardized, spacecraft configuration.