sorry kiddo,
i hate to say this but 100litres is not big enough for your bala's
or even really for a common plecostomus
i have never heard of "red tailed balas" before
and them living together for a while and "not fighting"
are the "red tailed balas" supposed to be red tailed black sharks?
if so they will live harmonously until maturity, then literally fight to the death one day
they are a solitary species
only one should be kept per tank
if they really are bala sharks then they are schooling fish so of course they will live happily together
bottom line is
your tank is far too small and nothing you say can change that
are you aware of their adult size?
obviously there is some research needing to be done
your fish will be stunted and eventually they will die if kept in such a small tank
you want at very
LEAST 130 GALLONS to house a school of balas