One way to balance cascade skills would be to make them less costly to acquire during play.
Digression:
During character generation, characters may acquire as many as six level 0 skills, as basic training for their first career. (That's if I remember correctly; my PDF books are on my computer, which crashed a couple of days ago.) They may also acquire or advance as many as two skills above level 0.* Based on six level 0 skills in four (or two*) years of intensive training, level 0 represents as much as 8 (or four*) months of intensive training, and advancing beyond level 0 represents two years of on-the-job training. College is a special case (and again my PDF books are inaccessible until I fix my computer).
So, to advance during play, the system should be scaled to no more than that amount of progression in skills.
But cascade skills are not necessarily as difficult as all-encompassing skills. Acquiring a skill with cascade skills requires learning both the general principles of the field and the specifics of the specialization. Examples: Engineering (Life Support) involves the general concepts of maintaining complicated machinery and the specifics of life support. Science (Chemistry) involves both understanding of scientific method (some of which may fall under Education) and the specifics of chemistry.
When one wants to advance a different cascade skill under the same general skill, the general principles are not necessary, only catching up with the specifics of the advancing cascade skill. Examples: advancing one's Engineering (Maneuver Drive) to match one's Engineering (Life Support) or advancing one's Science (Terragen Biology) to match one's Science (Chemistry).
Also, the specificity of a cascade skill (Terragen Biology is a subset of Biology, and a supeset of Mammal Zoology) doesn't change the difficulty of learning, but affects the depth of knowledge. Someone with Mammal Zoology wil know a lot about mammals, have some understanding of Terragen Zoology, Terragen Biology, and even non-Terragen biology. Someone with Biology will know things about Aslan biology, Hiver biology, Droyne biology, Terragen biology, and assorted other ecosystems' biology, and would probably be well qualified to study the biology of a newly discovered ecosystem, but wouldn't have much depth of knowledge about mammals, except that a scientist who happened to be a mammal would know more from familiaritywith one's own ecosystem.
The long Science example suggests that there could be a generalist cascades to some skills. For example, an engineer who was the sole engineer on a small starship might have Engineering (Small Starship), which covers all of Power Plant, Maneuver Drive, Jump Drive, and Life Support, but only on ships small enough to be maintained by a single engineer or a lead engineer and a part-time engineer who doubles in another department. The small starship engineer wouldn't be as good with a jump drive as someone with Engineering (Jump Drive), even if the latter trained on a giant ship with a hundred jump drive engineers rather than a small starship, and also would be at a disadvantage with any system on a large starship, but would be better with a jump drive than someone with Engineering (Power Plant).
One could even combine skillsthat aren't defined as subsets of the same skill. If I remember correctly, there are separate combat skills for melee, guns, and possibly heavy weapons, muscle-powered missile weapons, and unarmed combat. But some things about combat are true for an artillery piece, a shotgun, a javelin, or a fistfight: focusing on the attack, awareness of the need for defense, and the psychological ability to do harm to another sophonr. So one could define a general "Fighting" skill that covers it all, but leaves a character less capable at any specific for of combat than someone with a specific skill. (A generic "Fighting" skill could also apply to non-player characters, with a different meaning: they have the appropriate combat skill for whatever weaponsthey happen to be carrying when encountered.)
I'd also suggest the addition of in-training skill levels. For example, Astrogation-T3 means the same -3 roll as no Astrogation skill at all, but the character is studying it, Astrogation-T2 means a novice with a -2 instead of a -3, Astrogation-T1 is a more advanced trainee with a -1 roll, and finally Astrogation-0 and up have their usual meanings.
That's a long ramble, but the general points are that skills acquired in play can be different from those acquired in character generation, and advancing skills takes a certain amount of time of specific training or a longer time for on-the-jobtraining.
Additionally, a highly specific cascade skill acquired in character generation is worth less than a more general skill. Not balanced? Maybe not, but that can be treated as a case of "life is not fair", and that a character may just not have the best trainers or on-the-job mentors in some terms as in others.
* One point I was fuzzy about was whether one can acquire an above level 0 regular skill in a first term, in addition to basic training. Any citations that clarify that?
