There's a good reason that boarding isn't a major part of ship-to-ship combat, namely that it's much easier to fight to kill than to disable. And if you're looking to board, you REALLY need the enemy ship disabled first. A ship with working void shields is difficult to enter, one with working weapons will try to kill you on the way in and power armour isn't rated for macrocannon rounds. Never mind all the OTHER parts that might be used as weaponry - a sufficiently deranged heretic might try to take the ship into the Warp without the Gellar field.
You can find a (sort-of-friendly as the ship isn't shooting back) boarding action in the Shira Calpurnia novel "Crossfire". The arbites are in a small ship and have to get in first - which means matching velocity (to be boarded ship already forced to a near halt by the Navy), getting in which requires them to find an airlock, manage to OPEN the airlock, and get the sealing right to keep both your own and the other ship pressurized.
Not as directly important to Sororitas as it was to the Arbites as their Power Armour is environmentally sealed (if they wear their bloody helmets...) and will likely let you power through the newly created wind tunnel, but air leaking out will at the very least have damage control shove a LOT of extra doors in your path.
Anyway, keep in mind that the ship may well be multiple decks each kilometers long, so you probably want to find an entrance near your goal... assuming you even know where that IS. The Captain you're looking for? He's mobile, and unless he has a very good reason to stay in one place you have a LOT of searching to do. Most of it probably through crew fighting back.
That will get rather like city fighting, but for obvious reasons neither side has much chance for reinforcements or artillery support. Or heavy weaponry, because hitting the wrong thing can cause anything from a giant explosion to a hull breach (esp. noteworthy for the boarders, as you don't know the ship as well). Flamers and melee are your friend, bolters get iffy, meltaguns are right out unless you KNOW what you're going to be using as a backstop.
It's going to get messy. On the good side, a good part of the crew probably doesn't want to fight (or, if the ship is Imperial, might well join your side - what pious citizen would fight a Sororitas?), and you'll have an edge in armor, training, and faith