There's no official board for complaining about privately-run hostels. They're independent businesses in the same way that a small B&B or corner shop is.
If the hostel was part of a commercial or charitable group (such as the Scottish Youth Hostels Association) you can obviously complain to the management of that group.
If the hostel had a 'star-rating' awarded by Visit Scotland, you should report your concerns to them. Try an email to:
[email protected]
If you want to report matters to the local Trading Standards office, you can find the relevant contact details by entering the postcode of the hostel here:
http://www.tradingstandards.gov.uk/search/dbas e/searchlocal.cfm
Otherwise, you can only complain to the management of the hostel.
Chris