The trouble is that the loads you will be using before your 15RM will not provide much in the way of a hypertrophic response in most folks. Why not do just a week of 20s, and then start an HST cycle as per usual so you can try it out for yourself. Some folks like to do higher reps for their legs.
If you are worried about going as low as 5 reps because of the loads involved then try 6, 7, or perhaps 8. However, if you keep your form really good in all exercises then you shouldn't be using weights that will put you at risk of injury. It's when you lose your form that problems are more likely to occur.