Hi Flo
courgettes are fine
watermelon, any melon are fine (and skins)
I don't give Vit C as a rule as my boys eat a variety of veg but I did syringe water + crushed Vit C when GP2 was ill (he wasn't eating pellets and very little veg).
I saw a video where the owner was giving a Vit C tablet which the GP ate (not a human Vit C , IIRC it was an enriched vitamin tablet for GPs. And one very well trained GP). I'll try and find the name but it might have been in USA.
Mine don't entertain water so there's no point putting it in their drink.
There's not many veg that guinea-pigs can't have ; you just need to monitor the quantity.
Cabbage/brocilli/spring greens - too much can cause wind (and if they get bloat it can be fatal).
no ice-berg (too watery and no nutrition)
the very dark green veg like spinach, watercress , kale needs to be limited
fruit can give them sore mouths and you need to be aware of the sugar content
dandelions can be diuertic - so if you are trying to get fluids into the pig be careful how many she has.
I usually do 4-5 choices per meal and try to give different for supper.
Whole sweetcorn (with the leaves) is wolfed down here 
And GP1 thinks a sprig of parsley = a bag of parsley.