Can solar mounting parts from different systems be mixed just because they fit?

No. Matching dimensions or threads does not establish that an altered assembly meets its original structural, bonding or fire-classification requirements. Review the exact part numbers and installation instructions before substitution. IronRidge, for example, instructs installers to use its parts or parts it recommends; UL explains that rooftop fire classification considers a specific PV module and mounting system. Obtain documented confirmation for the proposed replacement and project design, including whether any relevant listing remains applicable. Different brands are not automatically prohibited, but do not treat an undocumented mixture as an approved system.

Sources: IronRidge, 'Roof Mount Installation Manual': https://files.ironridge.com/roofmounting/Roof-Mount-Install.pdf UL Solutions, 'PV Mounting Systems Certification': https://www.ul.com/services/pv-mounting-systems-certification