I believe he was talking about having a fairly high GF strength tech, which enhances their effectiveness. There are a number of methods for invading an enemy planet:
1. You can bring in terraformers and pump chlorine or some other deadly gas into their atmosphere. Note that this is probably the longest-term method, and that you'll need to remove it all later if you want to colonize the planet. Also, this will only kill civilians, and I don't believe that it is recognized as hostile action (and thus won't trigger surrender), so you'll still need to fight it out.
2. High-rad bombardment. There are special warhead techs that boost radiation while reducing power, essentially making dirty bombs that do less damage to industry, letting you wipe out large numbers of civilians, and ideally triggering a surrender without needing to invade. This is usually the method that requires the most recovery time, as the radiation and atmospheric dust will kill productivity for a loooong time.
3. Ordinary bombardment. You're going to be launching missile strikes at enemy ground forces. Your goal here is to do as much damage as possible to ensure success for your own ground forces without doing so much that you wreck the industry you want to capture. This tends to leave lower levels of radiation and atmospheric dust, as well as causing lower civilian casualties.
4. Unaided invasion. If you've got good GF strength tech and a ton of forces, you can drop a few divisions off and fight it out. Depending on relative strengths, this can be both more and less damaging to planetary population and industry as compared to a conventional bombardment, as ground actions have a chance to destroy installations and kill civilians. Essentially, you're going to want to go for the method which will result in the most rapid surrender, as your real goal with (non-RP) invasion is to take as much industry as possible intact, as well as preserving the population for tax income and (eventually) more no-terraforming, no-genemod colonization options. After the conquest, you can use tractor beam-equipped tugs to move any surviving shipyards to your own planets, if you so desire.
Re troop transport, you can also use the general-purpose design, where your transports have both TT bays and drop modules, simplifying logistics. You're probably also going to want to add a CIWS or two as soon as your gauss tech is good enough to make it useful, as that can help save them if they're ambushed by an enemy warship. Personally, I tend to use the bay/transport type as my general-purpose transport, as well as division-sized pure transports for moving large forces in preparation for garrison duty or large assaults.
"Dropship", from my observation, is used more in reference to the fast-movers you use for boarding actions, which are essentially a company-sized set of drop modules bolted to a high power engine. At the moment, boarding is very difficult to pull off without disproportionate losses, and is generally only used for special scenarios and LPs.
You can get both thermal and EM readings on NPR planets, and if you've got a bit of knowledge (or bother doing a comparison with Earth) you can get a general idea of their population (indicated by thermal readings) and industry (EM) (mainly, for both of those, IIRC). I believe that espionage teams can also give you that sort of information, though I might be mistaken, as I never really bother with them.