I like 2500's, I use the 1000's for ultra lights,....and the 4000's for the heavy lifting.
Me, too, like Jim Sr. says.
While I do use 1000s for my ultralights, other than weight or balance considerations, there is no disadvantage to using 6 lbs. test on a 2500 or 3000, heck, even a 4000 series spinning reel. The line peels off the same; it'll just take a whole lot more of it to spool it up.
And, a 2500 would likely be smack dab in the middle of the range for a reel you could use, for now, on your L or ML rod but might later want to move to an M or a MH. It'd be my choice as the most versatile of the reel sizes.
*** If you mean 6 lbs. braid, I'd add that I have had super luck using Sufix Nanobraid (not the same as Berkley Nanofil, another good light line). I have yet to break off some big bass and reeled in a big catfish using 2 lbs. on ultralight tackle. So strong. And, it is available up to 14 lbs. test which is about the same diameter as 10 lbs. Sufix 832 braid, another great and popular braid.
6 lbs. Nanobraid is likely the diameter of others in the 4 lbs. class, strong as others in the 10 lbs. class. What a line!