Different company, would not be able to use the ticket with them.

Tkts are at least US$140 each way at this time of year, best to try TG I think. Otherwise one can get a night train and ferry but quite slow (Bangkok to Ko Samui, Pha Ngan & Tao — Travelfish.org). TGs checkin operation at BKK is quite smooth, all at counter J19

I would assume by next year Thai Tiger might be operating the route...