Without knowing what type of map you're making, nobody is going to be able to give you any specific tricks about balancing. If you truly want something to be relatively balanced, though, you need to have your map played by a large group of testers of different skill levels, and continually rebalance and retest different beta (and potentially, release) versions of your map based on feedback.
You should just be able to more-or-less eyeball balance units when you're in the alpha stage. Number balancing is easy to go back and fix - what's much more important during pre-alpha/alpha is making sure everything works as its supposed to. Once you get to a beta version, though, you need to get testers to balance it, because realistically, there is no way that one person will be able to try anything or will be able to think of every creative loophole that an entire playerbase can think up.
Looking at the Twilight's Eve ORPG, it seems that the release version is build 1.14d, which means they probably had at least 14 major updates, and presumably many more minor updates building off of the then-current build. I would guess that much of those updates were to fix bugs, exploits, and balance issues discovered by the testing staff and/or community. So even after a game is released, there are definitely often issues with balancing that sneak by, yet if a game is good enough, it won't be "rejected from the beginning", because people find the core experience fun.
What you need to focus on at this stage, rather than making sure each unit is balanced properly, is getting feedback from some core people that you trust to give honest opinions on whether the gameplay itself is fun and interesting.