If you are just going to play around with it your good. In production though, there is much to consider, as there is all the additional gear and differences in signal processing involved, adding greatly to the cost of ownership of such an instrument.
As far as the number of windings: First of all there is a lot of snake oil in pickup design already, like "advantages of tall pickup designs" even though the detectable field of a vibrating string only reaches the top 1/3 of the coil in most conventional pickups, not the entire coil where the bottom 2/3ds are nearly inactive, and only add resistance. You will of course have a larger effective field with increase of string gauge (mass), so in effect, with separate cols for each string, you may want to have different number of windings and possibly differing magnetic poles for each coil. Now consider that a string resonates as a longitudinal wave, and excites the pickup where it crosses over the magnetic pole is a sweeping motion from side to side. The part of the winding in front and in the back of the pickup also have less to do with the field than the sides, so coil and magnetic pole shape manipulation come to mind. a rectangular or oval configuration with less windings will prove better than a round design with more as far as tonally. In the end as even with conventional pickup design there are many variables to work with, every one making one difference or another, and no set or best formula when human hearing and personal preference enters the equation. Experimentation will have to be your best friend there, although I can say that a ball park figure to start at would be to up the coil wire length of a regular pickup (which can vary quite a bit) and at least double it or more, then divide it by 5 (in your case). Taking it from there you will get acceptable results, and a good starting point to further exploration to further mold the output and it's sound.
As to cross talk: It is not necessarily that big of an issue in general, as the separation itself within the small magnetic field of induction is going to make a huge difference already in separating the signal and cutting down on accumulative wave distortion, and the tiny amount of cross talk is not going to be a major issue in the grand scheme of things.
The largest factor (the biggest difference achievable) in the sound of an instrument with magnetic pickups is not coil windings or magnetic field properties, but pickup placement, IOW Where the pickup resides within the usable space of the scale length. That is where you will find the most variance to work with, and with a separate coil for each string, the most room to work with.
So without giving you a 25 page essay with all of the mathematical formulas, only to have you find out that it does not jive with human hearing in the end, and is still only a starting point destined to not work out as planned, I hope this is of help.
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