All troops are fundamentally identical, marines and garrison battalions are the same thing. If you wanted you could deploy engineers into an enemy ship, though it wouldn't do much good. The combat drop module is needed for instant excursion. You can land troops on an enemy world normally, but you're probably going to have your dropship shredded. Drop modules allow for instant drops.
I think your viking design should have two ships. One heavy laser mounted on an engine to get at point blank range and blast a hole, and one fast troop ship. Your laser ship could also mount a microwave, as these fry sensors and fire control. If you knock out enemy firecon, they can't shoot down your incoming troopships, just make sure the firecon stays offline and isn't repaired!
Higher level armor allows for lighter armor. All layers protect the same, but will weigh less. So if you've got 10% armor by mass, you could add more layers for the same size. This makes damage calculations infinitely easier!
Your minimum troop transport size is really an engine, a tiny fuel, a drop pod, and a command station. That's pretty much the smallest you can get away with (unless it requires an engineering, in which case a tiny).
Troop transport bays: living quarters and garages for combat vehicles. Combat drop module: they're strapped into the orbital descent pods and ready to eject into combat. Keeping them loaded in the combat drop module causes a heavy morale penalty, so it's best to load them right before deploying.
AMM sensors: Resolution 1 at the biggest size you can afford. You generally want to detect enemy missiles a bit further away than your AMM range, so if your missiles get 20m km range, try and get like 30m km sensor focus, with how missiles move you an fire it when it's out of range and they'll move into range. And if you upgrade missiles you won't have to upgrade your firecon immediately either.