Depends on the material and size. The best solution is to have the plumber inspect the loop and tell you what connection to order. Since you're paying the plumber for the install he/she should be willing to inspect the loop and advise in advance.

I advise having the plumber cut the loop and install a three ball valve bypass then connect to the softener bypass. That way, in the event that the softener bypass ever requires repair, you'll still have (untreated) water to the house.

Remember, the Fleck 7000SXT comes with it's unique bypass and you only choose the connection while the 2510SXT has to have the bypass requested and also the appropriate connection. For the 2510SXT I favor the Noryl pass over the SS one regardless of what the seller says. And make sure you get a top basket and gravel under bed in either case and there should be no extra charge.