I've found it really doesn't matter how many troops are involved, unless the target planet has an enormous population. Even on "tough", the AI usually extremely crappy soldiering skills for nearly the whole game - even the evil races. Unless someone declares war on me early and comes at me with a bunch of transports, I almost never invade anyone before I've researched Shock Troops and built Tir Quan somewhere. My army is basically a steamroller.
Normally, I'd find this to be cheesy, but the AI does it too, so I guess it's a valid tactic. Invade with a single transport to exterminate most of the defending army. If you win, great. If you don't, you probably killed at least half their army. Send in the fleet to mop up.
If the planet has high population and low morale, go with information warfare, because that generally gives a nice boost to the size of your invasion force. Otherwise, mini-soldiers. Unless.....
Always check to see what's on the planet before invading it, especially when invading Drengin or Torian planets. If they've built half a dozen damn farms, use Tidal Disruptors. That'll wipe out most of the crap they've built without destroying the planet itself. Saves you the trouble of having to manually destroy all those damn farms.
About halfway through the miniaturization techs (expert, I think), you can build a troop ship out of a tiny hull with an advanced troop mod. You can't make it as fast as a cargo hull, but it will be cheaper (and quicker) to build and you can put a huge number of them into a fleet. If it's early on and your logistics are only at ~21, that means you can still invade a planet with 10 transports all at once.
You can also usually fit at least one weapon on these transports and it'll have a chance to survive an ambush. Basically, it's a lightly-armed fighter. Although putting on a weapon cancels out the $$ saving between tiny and cargo hull. If you're going to do that, use a small hull instead so you can fit a little more stuff on it.