1) During this period not every battalion did have pioneers and the number could vary according to how many men with the appropriate skills were present within the unit. They were not part of the regimental establishment officially until the French Revolutionary/Napoleonic Wars. This was part of the reason that the light company men in the AWI period – and sometimes all troops – were issued with hatchets.
2) On parade, they stood on the right of the line with the grenadier company; this mimicked the practice of the German troops with which the British traditionally served, whether in Europe or North America.
3) In the field, the pioneers of each brigade were normally grouped together at the head of the brigade and would march off first in order to clear any obstacles along the route. It's not clear if any/all of the regiments in Burgoyne's army had them as Burgoyne himself requested a large force of Canadian "axemen" specifically to clear the way as his army advanced south.
4) In certain circumstances, pioneers could be assigned to other duties – eg at Bunker Hill, a group of them joined the 35th's grenadier company in helping the artillery to move their guns; clearing the abbatis and ditches ahead of an assault on a fortification was another specialist task.
5) In battle, their role was not specified. By the time of the Napoleonic wars, I think they were usually detailed to help protect the colours, but since many British units did not take their colours into the field in the AWI, this was probably not a role they performed in that conflict.
6) You should also note that contemporary illustrations show them WITHOUT beards at this time; again, the facial hair tradition really dates from the Napoleonic period.
7) Unlike the "Black Pioneers" which was intended as a unit of labourers and craftsmen, the "Guides and Pioneers" unit mentioned by Rougeau was very largely NOT a pioneer unit in the artisan sense, but a specialist recce/intelligence gathering corps composed of Loyalists who were familiar with particular parts of the Colonies.