I've been tuning pretty low on my 6 string, Down to Drop B, A# and A.

The problem is the strings seem pretty floppy and its not easy to trem pick on some strings because of the floppiness.

Do seven string guitars have more tension on the strings so the strings feel tighter?
Yes, as they have thcker strings, but you could buy some .12 or maybe .13 gauge strings for your 6 string and it would play far nicer be careful if your guitar has a tremolo though, as they can cause problems with the extra tension.
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A 7 string will change your intervals on the higher strings, remember. It sounds like you'd only really be using it to tune low; a baritone would be a better investment for you.

Or just buy much thicker strings.
just get thicker strings if you tune down all the time. If you're in the C/B area .12 or .13 is nice and chunky
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