Is it ok if I use the same username (i.e. wallet address) for all my miners in a p2pool? And if my miners are weak independently, do having them use the same username help to avoid the situation of not getting paid because of too low hashrate?
Yes, you can point several workers to mine for single payout address. Their work will be counted together.
Oh and also, if you don't specify the difficulty in the username what is the default? Is it likely to drop below the not-gonna-give-u-any-coins limit?
Not sure about what is default - there vardiff algorithm and it will change constantly. But even if diff assigned to your worker will be less than minimal you will be paid only for shares that meet required diff.