Why not simply make your troop rating a function of both training and experience? You should be able to compensate poor training with battle experience and vice versa.
TroopRating = Training + Experience
Experience is accumulated by actual fighting: damaging and destroying enemy units, being hit and getting away with it, and maybe even just surviving a battle. The experience amount gained could also be modified by the odds of the engagement so that beating up weaker enemies won't give you as much experience than being bloodied against stronger ones. Also, some race picks could give you an automatic initial amount of experience simulating warlike races.
Training is gained by assigning troops and ships to training facilities that you can build. Technology advancements give you more training facilities and an individual ship or fighting unit can go through a particular training facility only once. Training costs money and takes time meaning that the unit is unavailable for a few turns.
Conjuring up figures to make an example:
Empire A didn't bother investing in troop training facilities and just conscripts as it needs. Originally it's 1st Space Fleet consisted of 80 ships. Due to heavy fighting it has lost 40 ships but in the process its ships have accumulated battle experience on the average 250 points. Some more and some less. Most of the ships in the fleet are thus of the Regular troop rating level.
Empire B has been mostly at peace since the beginning so its battle experience is basically non-existent averaging to 20 or 30 points. However, Empire B thinks military training is an important issue and thus has built barracks, military training camps, space academies, etc. After being built all ships are routinely sent through the above training facilities so that after consuming a considerate amount of money and turns the ships join the Space Forces with 200 training points. The troop rating level is therefore Regular.
In the above we have two different strategies amounting to the same result. Both empires get the same combat rating, but A does it by spending ships (constant battle) whereas B spends money and time (training).
Of course, after B's fleet has gone through a war or three its ships have got both the training and the experience. OTOH A got its troops initially cheaper and faster. Which is a better strategy I think depends on how you intend to use your troops.