Skrap, it is possible to install a bimini on a tall rig Catalina 25 with no modifications at all, but the boom will be 1' lower than the boom on a standard rig boat. If you buy a new, shorter mainsail, it'll cost you
$7-800., and you can install the bimini at the same height as the standard rig boat, but when you get ready to sell your boat, you'll be selling a boat with a bastardized sail, and that won't do anything good for the value of your boat. There's a third alternative, that I think is much better than either of those two alternatives. Your mainsail probably already has a cunningham installed in it, about one foot above the boom. If you have your sailmaker install a cringle for a flattening reef at the same level on the leech of the sail, you can use the flattening reef and the cunningham as a sort of short reef. Then, when you raise the mainsail to the top of the mast, the foot of the sail will be about one foot higher, and that will provide more clearance for a bimini. Except for the most recently built C25s, the height of the boom is adjustable.
The advantage of this method is that your mainsail is capable of being used in its full size. If you wish, you can lower your bimini in light air, shake the short reef out of the mainsail, and use the full-sized sail for racing or cruising. By raising the boom one foot, your boom will have the same amount of clearance as a standard rig boom, and that will provide plenty of room for a bimini. The nice thing about the tall rig boat is that the extra sail area makes it much more enjoyable to sail in light air. If you cut your sail down, it permanently destroys that advantage.
For daysailing and casual cruising, sailing with a short reef in the mainsail won't hurt the boat's performance significantly, or make it tender. By comparison with a standard rig boat, it will still have a bigger jib than a standard rig boat, but the size of the mainsail will be about the same as the size of the standard rig mainsail. Your mainsail won't be carried any higher than if it was sailed without the short reef. It'll be carried at the same height, but the boom will be one foot higher, so there'll be a little less sail area at the foot. That certainly won't make your boat more tender than any other tall rig C25 sailing without a reef.