This is an interesting idea. Having a much smaller number of scientists and engineers, but making them cost much more and be "real people" that you can name and use on missions/base defense would be a cool feature. It's more realistic too in the sense that you'd have maybe a 3-10 scientists researching that alien corpse ... not 150. Seriously, how do 150 scientists research a corpse?
It sounds like it could get very complex with the physics/biology/etc specifics, but a "simple" way to do it would be 3 classes with 2 extra stats added, "Engineering" and "Science." Soldiers would have 0 for both of those, Scientists would have a number 1-whatever for "Science" and likewise for Engineers.
Anyway, probably not something that would happen, but a cool idea nonetheless.
Thinking of... an idea: civilian units(engineers and scientists) in base defense mission. Not all 200 of them, of course, but 3-5. But how to make killing them count as decrease of amount of personell...
This seems like the most realistic implementation of this, just using the allied civilians mechanic. But rather than have 1 civilian be worth 10+ scientists/engineers, you could have them count as one and cap the total number at 10 or something. Those would be the ones that didn't evacuate in time or something.