You wouldn’t tend to add sleepers afterwards. Considering a sleeper wall is constructed, then S&C, then DPC then more S&C and then a section of 4” x 2” on its side which is then screwed/nailed into from the joists above- I can’t see how adding a sleeper wall would work easily after the joists are down.
Take noggins out and double up the 4 x 2. It’s fairly simple and cheap. Where are you getting timber from locally
That’s because they’re completely different products.
And have different functions.
Bps are great. Don’t use anyone else