on the su-hs6 carbs, if the idle is lower does that mean the car runs less rich or richer?

There may be a tendancy for a richer mixture to lower your idle.
But I would not use that as a guide to determine if your carbs are in tune.

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'60 PV544, '68 122, '70 145, '84 Prelude, '86 745T, '06 MPV

All things being equal, if you are running rich at tickover your revs will increase as you weaken the mixture by screwing up(sic) your jets. When it starts "hunting" back it off a couple of flats. Your revs may well have increased by about 200 at this point. If your tickover is smooth and it seems smooth when driving, the carbs might be fairly well balanced even though the mixture is way off. If this is the case, make sure you adjust each carb equally to keep this balance. If it all goes pear shaped you'll just have to get the book out and start from scratch.