Stay away from te 3/8 pex off a manifold, for one thing, yes I have encountered the 3/8 as the main lines out to the fixtures, it was quest pipe, if you know what it's like to fix a leak on 3/8 in the wall and almost no way to make the transition from it to pex, almost impossible, supply house will have no way to help you in the case, I did not warranty my repair on it, I had to use a 3/8 comp x 1/2 comp quest coupling.
Just don't use this crap. I have only found it twice here and once in the walls, no way to replace a run unless you take it all the way back to the block, this was not an option.
It was not an older house.
I routinely use 3/8" pex water supplies for sinks and toilets. Still, 3/8" size of any pipe is not allowed inside walls as per the latest code.