  Incorporation of the standard model Yukawa interactions in a grand unified
theory (GUT) often predicts varieties of new scalars that couple to the
fermions and lead to some novel observational effects. We assess such a
possibility for the colour sextet diquark scalars within the realistic
renormalizable models based on $SO(10)$ GUT. The spectrum consists of five
sextets: $\Sigma \sim (6,1,-\frac{2}{3})$, $S \sim (6,1,\frac{1}{3})$,
$\overline{S}\sim(\overline{6},1,-\frac{1}{3})$, ${\cal
S}\sim(6,1,\frac{4}{3})$ and $\mathbb{S}\sim(\overline{6},3,-\frac{1}{3})$.
Computing explicitly their couplings with the quarks, we evaluate their
contributions to the neutral meson-antimeson mixing and baryon number-violating
processes like neutron-antineutron oscillation. The latter arises because of a
$B-L$ violating trilinear coupling between the sextets which also contributes
to some of the quartic couplings and perturbativity of the same leads to strong
limits on the sextet masses. Using the values of the $B-L$ breaking scale and
Yukawa couplings permitted in the realistic models, we derive constraints on
the masses of these scalars. It is found that $\Sigma$ along with any of the
remaining sextets cannot be lighter than the $B-L$ breaking scale,
simultaneously. In the realm of realistic models, this implies no observable
$n$-$\bar{n}$ oscillation in near future experiments. We also point out a
possibility in which sub-GUT scale $\Sigma$ and a pair of $S$, allowed by the
other constraints, can viably produce the observed baryon asymmetry of the
universe.
