8 string sounds really fussy to me. Cool I suppose if you want to devote a lot of time and probably money into a unique instrument. I'd rather have a solid 4-string bass that I could play normally, with an octave doubler pedal, and with a whammy pedal to get you up an octave. More flexibility that way.
Especially if you don't have a bass currently, I'd definitely push you towards getting a regular bass and then doing weird stuff with pedals. A regular bass will serve you forever, while an 8 string bass will fill its niche but won't do a good job as a regular bass. I can see parts of a song benefitting from the octave doubling... but personally I'd want to enable and disable that effect.
Coming from guitar, a regular 34-35" scale bass can seem big, but you'll be surprised how quickly you can adapt. Just play the thing for a couple of days, give yourself time to get used to it and your hands will remember what they're supposed to do.
Mentioned this in another thread, but I have a Squier Bronco bass (30" short-scale) and it totally rocks. Sounds every bit as good as my Fender P bass in a band setting. Build quality isn't as good, but the intonation is great and the tone is solid. Getting made-for-short-scale strings made a lot of difference here (they have different tension ratings than regular bass strings). Easier to get around on coming from guitar. But again, if you're rocking a 7-string guitar now, you're already used to bigger necks, so I'd be confident that you could make the switch to a full scale bass. Something like a Jazz Bass will have a smaller neck than a P Bass.
Piccolo basses are another niche, sort of random instrument. Cool because they're different, but not terribly practical. Again I'd point you towards a regular 4 string bass, and you can put piccolo strings on there at any time.
So if it were me, I'd get a 4 string bass, an octave doubler pedal, and then see if that met my needs.