You can always invite Slaine to stay as part of the crew now, and they are free to do so, but that's a PC choice. Another option (that doesn't involve Slaine working but can get you some credits) is Slaine renting a berth as a passenger instead of a spacious room in the station. You will have to talk about it with them, either way. Standard fares are now available in the Game details.
Mariella and Stone are paying in the background, so they don't count as crew. By the way, you have empty berths, feel free to sell passenger tickets if you wanna risk it, after you talk with Valeria about it.
Hank asks Erin, "Did they say why they quit en mass? Just didn't want to be part of a cursed ship, or was there something else driving them away?"
1. Nameless NPC crew costs 100 credits per day, per person, in wages (time in cryo-pods doesn't counts) plus any signing bonus (like a passenger ticket home). Valeria kept some of that money for the crew. PCs have "special arrangements" that can be talked about In Game, but we don't need to and can gloss over that. (ie. we usually skip this minutiae for PCs, as that's boring, but we also don't deal with small amounts of daily expenses, bar tabs, small luxuries and cheap equipment for them as a consequence)
2. Crew (PCs included) needs 20 credits per day in food and regular supplies. This comes from the ship's money, not from yours, in bulk, whenever you resupply. The character sheet calculates that for us.
3. You have to pay 344,375 in maintenance every 6 months. This is automatically calculated by the sheet. The next one is in 5 months from now.
4. You may need that 96k for speculative trading, so you get more money out of it. Specially because you have an empty 2,000 (!!!) tons cargo hold now. The Explorers Division hired you for a large part of that hold, but not until you reach Altairia.
PS: You can also use those holds for "sealed crates" (ie. freight) with some local broker or company. Standard rates are now available in the Game details.