Hi Chris,
I love the idea of stopping in Orisson for your first night. They actually have tents available behind the gite, for a slightly lower price than the dorm beds. The tents sleep two people each, and they are really comfortable. If you'd like this option, mention it when you book (
http://www.refuge-orisson.com/en/auberge_compostelle_pays_basque_contact.php)
The other choices out of SJPP are Honto (5 km) and Kayola (7 km). Both should be booked in advance. Kayola bookings are done through the Orisson website. I've never stayed at Honto, but here is some information:
http://caminodesantiago.consumer.es/albergue-ferme-ithurburia Orisson is 8 km from SJPP, and after that you will need a tent if you want to stop before Roncesvalles.
Believe it or not, but the statistic from SJPP show that fewer people start from there in August than in the busiest months - May and September. That doesn't mean that the entire way to Santiago will be less busy, as I think things bunch up from Sarria to Santiago. Nonetheless, you should be able to find beds at the albergues.
One tip, if you are game to get a Spanish SIM card, you can make advance or same-day reservations at most of the private albergues. You can put the SIM card into an unlocked phone from another country, or you can pick up an inexpensive flip phone at the shop where you buy the SIM. The company that most often has English speakers is 'Orange', and the first one you will come to will be in Pamplona.
If you don't speak Spanish, you can always find someone to make the call for you, using your phone.
If you do decide to camp, yes, you can do it along the Camino. I've never done it and am not an expert (I'm more of an 'inside' girl!), so hopefully someone else will add some input on that.
As for sleeping bags, yes if you will be camping. If you will be staying in albergues, you will need
something, but I highly recommend the silk sleep sack over a traditional (even lightweight) bag.