7 pH is neutral.
As mentioned previously there's nothing wrong with adding oak leaves if they have been dried and don't have any disease (fungus, etc.). The pH range for Corydoras paleatus is listed as 8 - 6 pH so a handful of leaves certainly isn't going to lower you pH that much.
When you first put them you may find that they float. I place a small rock on them to keep them down. Don't use a rock that's affected by acids like limestone. Smooth / rounded granite is great. Even after they become water logged you may still need to weigh them down, especially if you have a strong water current.
It's hard to say how long to keep them in, it depends on their age and how much wear and tear they get but I'd replace them once they start to break up.
Also as mentioned, as the tannins leach out your water will get a tea coloured tint. Nothing to worry about.
It's a nice idea and I've done this myself in the past.
Enjoy.