All unit sizes are available from the get-go, because there are no longer battalions/companies/brigades/divisions. What unit sizes players use is entirely up to them. Marines are infantry with the boarding combat special ability and all special abilities are also available from the get-go, if I'm remembering it correctly. So you can create a power-armour marines with genetic enhancements and personal weaponry and deploy them in squads of ten if you want. You don't need the boarding combat special ability either, but it makes things easier. Since the size of boarding units is restricted - you can't hardly expect to drop 10k grunts - it's better to have them be as high quality units as possible. You can have them board a ship with their own HQ as well, to get more combat bonuses.
Here are some relevant quotes and screenshot:
The smallest troop transport bay is 100 tons or 120 tons with drop capability included, so you can make very small (fighter-sized) ships to drop off small formations.
Is this a new type of transport specifically for boarding or has it always been like this and I just never noticed?
It is a new type - discussed in some of the C# threads but not in the changes list yet.
Based on comments above and my own play test, I have changed how HQ capacity works. Instead of multiple HQ components, there is now a single component with a configurable capacity (similar to STO in principle).
You select the HQ component and then type in the required capacity. The component cost is Capacity / 2500 and the component size is Capacity / 50 with a max of 500 tons. There is no limit on cost.
Because of this configurable aspect, the HQ can only be placed in the primary slot for those units with multiple slots.
Note the BOARDING COMBAT special ability, only for infantry, on the top right segment:
Post about Genetic Enhancement:
http://aurora2.pentarch.org/index.php?topic=8495.msg112049#msg112049The game mechanic for Boarding Combat:
http://aurora2.pentarch.org/index.php?topic=8495.msg111751#msg111751Also, one thing to keep in mind is that the research cost for Maximum Engine Power Modifier has been halved, so it's easier/cheaper now to research high-speed, boosted engines for your boarding shuttles.