I've never tried building a pressure (including vacuum) style DIY filter out of a
barrel but I don't see many other people do it either and figure there must be a reason.
I don't think a large rock filter alone will do much other than pass water?
I'd agree that two smaller s/g filters would be better but I'd plumb them in parallel. Two
won't work in a series as it takes about 17" of head to gravity feed the second one to make
it not overflow the first.
Another option might be to make one s/g filter and have it feed a small shower/trickle tower
style design.