WYSIWYG

Conversions, conversions! You could represent the twin-linked missile launcher by a big targeting lens or something where the second weapon would be. You just have to explain to your opponent what you've got. I try to hold myself to a tournament-standard wysiwyg and tournament-standard of painting (i.e. I don't use unpainted models) but I obviously can't hold anyone else to that standard since I'd hardly ever get to play if I did.

Generally, things that the squad can have as a whole just needed to be represented on the majority of the models. For example, the three meltabombs on the assault sprue. Three of five is the majority. For meltabombs, I only model one or two in the squad with them, because you have to figure that's pretty specialized equipment (not to mention I never actually take it, so it's not really an issue anyway) so you wouldn't arm an entire squad with that sort of thing.

As a general rule, even when everything is WYSIWYG, it's good to let your opponent know what you've got on each model if there's even a negligable possibility that it could be mistaken for something else (i.e. frag grenades and krak grenades look a lot alike when you're not looking closely at the model.)