Love Highland has 2x set "Romantic Hosts" (married owners of the house) and 2x set "Singles". Everyone else (6x "Singles" by default) has the Role Criteria: Age (YA-E) and Relationship Status (Single). You can change out selected Sims for those of your choice. Any empty slots will be filled using Role Criteria.
It also pulls in Sims with new Role Traits (Seasoned Romantic Host/Seasoned Single), which the Hosts and their two Single roommates have. These Traits aren't available in CAS, but can likely be cheated on to Sims if you want to see them all the time (maybe give Don Lothario the Seasoned Single one, for example). These traits have filters and can be set as Role Criteria.
"No Strings Attached" negates jealousy for all Sims on the Getaway. It doesn't work for non-Getaway Sims invited to the lot, or if anything happens after the Getaway ends. There are no new romantic interactions tied to that Rule. The "Love is in The Air" Rule does give a strong/almost constant Flirty Moodlet, so is worth using to prompt more Romantic socials during any type of Getaway.
Each Role can have 1-8 Sims, but you can use duplicate Roles if you want more Sims to do the same thing. So if you duplicate/add an additional "Singles" Role it would be called "Singles 1" by default and the Sims will be available for any of the Rules/Timeslots where "Singles 1" is included in your Schedule. There is a built in limit of 20 Sims total on the Getaway lot - if you are running it on your home lot, this includes your family members. I'm sure mods like MCCC will allow for this to be increased, but performance may vary based on your computer specs.
Elimination Getaways end either when the Getaway is planned to end (number of nights set/extended to), or when it can no longer run if there's another Elimination. Something like the Hunger Games/Great British Bake Off with Skill-based or Player Choice Elimination can run until there's only one Sim left. Social Elimination like Love Island/The Bachelor/Big Brother (Romantic or Popularity Elimination) requires two Sims to remain, so they can socialise with each other.
Love Highland default lot has a double room off to the side (for the married hosts, I assume), two twin rooms with two single beds each, two double bedrooms and one room with twin double beds. So there is room for 8 Singles to have their own single or double bed, but some will be sharing rooms. If you have more than 8 Singles by duplicating the Singles Role then some Sims will miss out or have to share with a partner. If you don't have enough places to sleep you get a TNS warning/prompt when the Getaway begins.
Hope this helps.