Since the abstract boarding rules (TO p201) would probably be used to handle boarding during an ongoing aerospace scenario, the actual equipment used by non-battlesuit marines doesn't matter, other than body armor with a divisor of 2.
For short term boarding flights, you can carry 5 non-battlesuit marines per ton of cargo space (TO p199), no bays or quarters needed, so any small craft could be used for boarding actions. If using battle armor, then it'd be 1 per ton no matter what size (TO p199) and given that they're only worth 2 marine points (Mk XXII, or Achileus/Longinus config with flamer or MG) or 1 marine point (other Mks or Achileus/Longinus configs) each, then per ton non-battlesuit marines are a better choice with both divisor 2 body armor (2 points per marine) and other body armor (1 point per marine).
Some Nighthawks could be added for flavor, but I'd suggest just assuming that all the SLDF marines come with a Marine Environment Suit (thus 2 marine points each). Weaponry is unimportant, but would probably include a lot of Mauser 960s given that it was the standard SLDF infantry weapon. The Mauser 960 would also likely be wielded by Nighthawks.
The Achileus is probably more likely for boarding actions by the FWLN if they're going to use battle armor, as its smaller size would make it easier to get through cramped passageways, although the rules don't place any such limits. As noted above, whether Achileus or Longinus is used is irrelevant, with the weapon config being key. The FWLN are also likely to use Marine Environment Suits for their non-battlesuit marines, but they're typically only armed with spoons.
Assuming the OP ships, the FWLN have 8 small craft, while the SLDF have 34. So potentially the SLDF could deliver a lot of marines, with each marine as effective as the battle armor available to the FWLN at the time.
I might have lied about the spoons, although it doesn't matter as far as the boarding rules are concerned.
