On Bitfinex you buy Miota, and yes iota is like satoshi the smallest part.
The seed well you need to create it in this way in linux:
cat /dev/urandom | LC_ALL=C tr -dc 'A-Z9' | fold -w 81 | head -n 1
or use a password generator, but only capital letters and number only "9" and length 81 charakters
this will be a string like that:
UF9IEMCLEN9...................................EI9WEF9DER9  <- 81 chars