1. Would a P90 in the neck and a single coil in the bridge cause any conflicts?
2. What pots would best suit - 250k or 500k? (I will probably replace the stock ones)
3. What would be the best capacitor to use for the tone pot?
4. I was thinking of contouring the back of the body similar to a strat. What would be the likely impact on the sound?
5. If I do contour the back what is the best way to approach this? (I have not attempted this before so would probably need to buy suitable tools)
1 - You want to make sure the P-90 is the reverse polarity and reverse wound to your bridge pickup if you want the middle switch position to be in phase and hum-cancelling.

2 - A lot of people use both for P-90 fitted teles. A 500k for volume and a 250k for tone. It's ultimately what sounds good to *you*.
As Mark said, control cavity will require widening if you use 24mm pots. This may open a can of worms but, there's nothing wrong with mini pots if they're good ones. It's more about the quality of the pot, not the size IMO. So if you're not prepared to widen the cavity, maybe look at buying good quality minis.

3 - Again, tone is subjective, so it's what you hear & like, but the most common cap with teles is .047uf. I have typically used .047 or .033 with good results (for my ears). FWIW don't get caught up the BS about "orange drop" or "vintage" caps. Cheap, modern ceramic or poly caps ARE JUST FINE.

4 - I contour ALL my guitars. It just makes sense to me and feels so much better IMO. I would argue that there is NO impact on tone. Whilst I agree with *some* of the debate about tonewood mass/density affect on tone and sustain, I don't think the amount of material removed for a contours is that relevant.

5 - I use either a drum sander (80 grit, in my drill press or corded hand drill) or a flap disc on my 4" angle grinder. Just beware that basswood is quite soft so you want a light touch as material disappears quickly.

Hope this helps. Look forward to seeing the progress. Keep posting pics!

Cheers,
Mick