If you optimise for more than one goal, you will (ideally) end up with a family of solutions rather than a single one. You then still need to either manually or programmatically select one specific instance out of this family before you have your actual truss shape.
It is true that Galapagos has no tools for exploring Pareto fronts, but are you sure that you really need to optimise along multiple axes, rather than optimise for a compound fitness function? I.e. minimise weight, while maximising strength, while minimising the number of beams that exceed some length range at the same time?