Thanks for asking about the SkiMo training plan. If you have got a good aerobic base then you can can certainly start to do more focused racing intensity workouts 1-2 times per week. The plan is laid out with an 8 week transition period then moves into 8 weeks of more SkiMo specific workouts with intensity and lots of vertical. If you have also been following a a good general strength program then you could use this plan effectively for your March race. I would do a couple of the 8 transition weeks and then move in the remaining more specific workouts. These workouts can be done on snow just fine.
As for your separate question about doing long ski tours during this plan: If they are not so draining that you can not complete the other workouts then by all means do them. You will find that they best way to accomplish this mix is to keep the intensity low on those long tours.
If the ski tours are long and hard(ish) then you your training will not be as effective.