It's easy to catch a zealot. So what if one can take out two marines, how about 3 marines. lol.
anyways...
I don't think hybrids would be an INVINCIBLE race if the terrans were the ones doing the experiments, or even the Xel'naga.
First you gotta remember the Xel'naga were the ones who created the races. Now we have the crude terrans doing "experiments" on these two races. The terrans don't have the blueprints of either species, or the knowhow of creating either in the first place. They're just messing with an already established base, taking them and trying to create a powerful weapon that can be handled.
Secondly, mixing two species wouldn't create a species as powerful as the sum of the two, but just having the characteristics of the two parent species and if genetically enhanced, the more desirable traits of the two.
For instance, you mix a cat and a bull, you don't get a huge powerful bull that can leap 30 feet and moves around like a cat. It just wouldn't be possible. You could have a huge powerful bull with claws and fangs and such, but not the agility of the cat. Or you could have a tiny bulllike creature with claws, fangs, and horns, that has a powerful little body yet still with alot of agility. You just can't have it all.
Perhaps the Xel'naga created one with purity of form and the other essence cause they couldn't create a single species with both. Maybe the Zerg were wrong in thier thinking they'd be "perfect", maybe they'd just be complete in a sense. Just as you can't have an incredibly agile bull that has the strength, or a very powerful cat that still has great agility. You gotta give up some for the other.