TOW has a large variety of cones, and lists all their cones with the thread size, and also note which guns they will fit, or should fit. As far as cap fit, I found that the Stainless cones that TOW sells fit the CCI#11's perfectly. My idea of a perfect fit is when after the cap is seated on the cone, you can't get them back off with just a fingernail. (or I can't) That keeps the revolver more water proof for field use, and they can't fall off from recoil. That may not be ideal for others. ?
I prefer the stainless cones over the Slix-shots, but that is just me. Not saying they are "better", just that they work better for me, the cap fit is better. (for my purposes)
Yes length is important as Dave says, although there's a little leeway there.