can't offer any suggestions for a hotel or company, but would suggest making your enquiries very soon.
Most hotels of the size you need seem to have rooms allocated to different operators, so to find one operator with, I imagine 20-25 rooms at the same time may be difficult after people have started booking for the season.
Good luck

I wouldn't rule this place out, seems very big for you all. http://www.highlandbeachresortgoa.com/home
Based in Candolim and very close to all you want, only 10 mins to the beach (would love to see the shack owner's face, whichever you go for !!)
Have you thought about going DIY ? might get a better discount all round.. Not sure which travel ops go to HB.

Lots of the apts in the HB are privately owned, so possibly one operator couldn't offer enough.

Cheers del, and reading the other thread that mr stella has posted in about the wedding is it worth it ? Seems a lot of hassle and not to mention the stress..