Soup is very forgiving... I just put in whatever I fancy and it's always lovely. No two batches ever taste the same!
I am going to attempt to make butternut squash, red pepper and red chilli soup this evening. Should be interesting. 🙂
What sort of chillis are they? Cos they vary in heat!
Depending on how big your pot of soup is, I'd try half a red chilli chopped very finely at the beginning, and then add some chilli powder and black pepper at the end if you think it needs more of a kick.
If it does end up too hot for you, it is retrievable by cooking some carrots in stock, liquidising and mixing into the spicy batch.