No they're not limited for passholders. As long as there are tickets you can book passholder fares.

However with 7 you'll likely have to share with others: with a 1st class pass, 4 together + 3 and 1 stranger. With a 2nd class pass 6 together and 1 alone. Something like that.

You can check availability for these reservations on https://www.raileurope.com

Use the desktop, not mobile, website, and add the pass in the search box.

It should be €20 per person for the couchettes.

It should, somehow, be possible to book 2x 4-person couchette compartments using Espace Privatif. 

These are bookable via Raileurope, the interrail/eurail reservation service or NMBS, Raileurope doesn’t have any booking fee, NMBS has a €4 booking fee per order, interrail/eurail has a €2 per person per train fee. The first two only require a pass number when paying for them, not for looking for availability. So this way you can check availability, and if you log into raileurope you can reserve these beds for a week I think? By keeping it in your basket. 

It isn’t on raileurope, but adding another person and entering one of your own pass numbers also works

Thanks for your help, I really appreciate it. Can you explain again how on raileurope, I can book 2 x 4-person couchette - I wondered if I should book 4 people in one booking, then 3 in a separate booking? But would be nice to have the rooms close together.

You just add 8 passengers and make one booking in one go

Be sure to double check before paying that you are in the same compartment! You can recognise this if the first “seat” number is the same, and the carriage number. 

for example; this is a full first class compartment that I have booked: 

for example; this is a full first class compartment that I have booked: 

 

Thank you for all your help!