The Planck scale might or might not be the "smallest" scale. Wikipedia gives the Planck scale as:[math]\ell_P=\sqrt{\frac{\hbar G}{c^3}}\approx 1.6\times 10^{-35}\ \mathrm{meters,}[/math]but this is just what you get when you look for the simplest way to construct something that has units of length using some of the dimensioned constants in QM+gravity, it's not in itself part of any theory. An actual theory might reference the Planck length multiplied by any dimensionless constant whatsoever (for example, e-137.035999=.306テ--10-59 would be nicely small, where 137.035999 is the inverse fine structure constant, which is dimensionless, but of course any such number depends on details of how a theory works).
If a specific theory, even something much more accurate and much more beautiful than the Standard model smashed with general relativity, says there is nothing smaller than the Planck length, that is a theoretical statement that might be supported by experiments for a few centuries or a few millions of years, but we can never be certain that some future experiments won't demonstrate that in fact there is a much smaller scale. Even if in fact there is no smaller scale, we can't be certain there is no smaller scale. One can have theories, this is not a counsel of despair, but nonetheless one has to be cautious of hubris, IMO, albeit that also includes that one shouldn't be too attached to lengths in a 3- or 4-dimensional geometry as necessarily absolutely important, perhaps something else altogether yet to be imagined is the measure of what it's really at.
In philosophy of science this argument, more-or-less, is called "the pessimistic (meta)induction" (at the link, reasons are given that hope to limit the consequences of the argument for scientific realism, but see also Structural Realism for a different response, or one can be an optimistic anti-realist[The theory might not be real, or it might be, but so what, we can still do stuff, we can even spray stuff even if it's only theoretically real]).