I think it really depends on your story and also the looks of the unit. Or for whatever reason, if RPG, a zergling may be described as "human" maybe because he was previously a human and changed somehow (magic?) or maybe even that they wear costumes of what they see themselves as.
As for strengths and weakness of each unit and balancing, I'm also having trouble with that. I am looking at fire rate, range, type of damage, and special abilities.
I find a lot of maps either screw up by overpowering one unit over another, especially the marine, or realizing the marine could be too powerful, so they make it very weak... Also the combination of the team(s) that may be involved in a battle at the same time. I also included Roles, as in is the unit a meatshield, or does it mainly do damage with low defense? Or maybe it's a balanced unit that's decent with both but does not excel in anything in particular?
So I have a bunch of variables which I then wipped up a formula (of which isn't really working, but the variables still exist)
Range - The unit with the shorter range is likely to be attacked more
Damage Type - There are different kinds of damage, Concussive and Explosive, for example
E indicates Explosive Attack (50% damage to Small Units 75% damage to Medium Units, full damage to Large Units).
C indicates Concussive/Plasma damage Attack (50% damage to Medium Units 25% dam to Large Units).
S indicates Splash damage, which affects all units in the blast area. - www.battle.net
I don't mind a unit having Explosive Attack, so I don't add or negate a units damage based on that, but for Concussive damage I do add more damage as it basically sucks because it goes all the way down to 25% on large units. There is also NORMAL DAMAGE, where the size and type of armor the unit has is exempt, meaning if the normal damaging unit is to hit 50, it will hit 50 regardless of unit type. The marine is, for example, a normal damage unit. And because of normal damage, I might add a little to Explosive attack, and a bit more to Concussive.
Splash - I generally cut down a units damage for RPGs due to their ability to splash...
Ground-only: Some units that are ground only deserve a little bit more damage in RPGs, IMO.
Special Abilities: Special abilities like stim makes the marine and firebat fire almost if not twice as fast, so I do a major cut in their damage.
Fire rate: The fire rate of a unit is a major factor... I personally have to be careful with this one, because while the Marine (15 cooldown) technically fires twice as fast as the Dragoon (30 cooldown) should not mean the dragoon should do twice as much damage as the marine. Why? Because while the marine is hitting a unit twice as fast, the armor is also calculated twice.
So we have 3 units:
Marine, Dragoon, and Zergling.
Marine does 30 NORMAL damage
Dragoon does 60 EXPLOSIVE damage.
The Zergling has 20 armor and 200 HP.
One shot from the Dragoon would cause the zergling to lose 40 hp.
Since the Marine shoots twice as fast as Dragoons, in the goon's 1 shot, the marine would have attacked twice, but the damage would only be 20. The first hit would be 30-20 = 10, the second the same, therefore 20. But also the Dragoon actually only cause 50% damage to small units due to its explosive attack, so the 60 is reduced to 30, which means they actually do the same amount of damage at the same rate...
The marine then still has the potential to stim, but even attacking faster with stim, one still has to keep in mind that whatever damage he is doing is still negated from the armor each time.
Then there are personal ideas, such as Roles.
If a unit in a group is meant to be a meatshield, maybe he'd get a bonus in armor, but a penalty in damage. If the unit is suppose to be a damage dealer (DD), then maybe it'll get a bonus in attack but penalty in damage (perhaps best with ranged units). Then also if you want some units that are balanced in both, but does not excel in either. Anyway, personal preference and creativity could also bring extra twists to the balancing system. Even the frequency of battle and the amount of teamwork you'd like in the RPG would determine how you would want it to go...
Once you've created a formula with all these (and more), apply to all units, and give an edge to heroes perhaps by giving them more hp over the monsters or simply allowing them to upgrade. To prevent the bad guys being as strong as the heroes at the beginning, maybe you could modify hp percentage and start them out at different levels up upgrades.
Anyway, these are just what I'm considering to balance my map and I myself have not found a fitting formula that will work with the amount of fighting frequency and to maintain balanced as the heroes get upgrades and gain HP by leveling up.
So here are what I observe, let me know yours so I can take it into consideration on my own map.
Thanks
None.