There is no middle position SSL-6, only a standard and RW/RP. Unless you plan on splitting the Pegasus so the screw coil is active and combining it with the SSL-6, I see no reason to get an RW/RP model, and if you are splitting the Pegasus I would go with the SSL-6T model.
SSL-6 is good for single note leads, but has a lot to be desired for a traditional single coil sound.
Maybe you aren't looking for a traditional sound, but I can't tell. If you're going to put a darker pickup like an SSL-6 in the neck, you might as well go noiseless since all you usually lose is the top end. Some complain about noiseless single coils not having the "feel" of a true single coil, without going into any detail about what that really means, or consider that this impression may be due to some other reason than merely the general concept of "noiseless" which can be obtained through a variety of methods.
As far as output goes, I think the SSL-6 is a tad hot in the neck against a Pegasus.
...and FWIW, I had an SSL-6 in the neck position of one guitar for two decades.