A core element of construction rules in BattleTech is that, for vehicles made using a tonnage scale, you round up to the nearest half ton. So in this case, 1.5 is a valid value.
See also p.22 "When rounding weight values for units using
the tonnage standard, a designer should round up all weight
values to the nearest half-ton unless the unit’s construction rules
specifically state otherwise."
You may also be interested in the Fractional Accounting rules in TO, which specifically ignores that 0.5 ton concept. (Although in the case of the Pompier, it'd change nothing with regards to environmental sealing).