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What are the rules regarding substituing characters that dont have models made for them?

It all depends on the To of the event. Usally though you need to convert/kitbash something that is WYSIWYG, and everyone is fine with it.

At my local it's pretty much the rule of cool.

Technically you could bring a baseball to represent a tervigon 'legally' as long as you remembered to glue the biomorphs to it, but most tournaments have a minimum percentage GW stuff rule to avoid this kind of travesty.

As long as the unit has no 'official' model, base size or look (aside from very dodgy codex art -voidraven for example) there's not much they can do. It is of course the TO's right to make judgment calls at whatever tournament you're at, so ask first before using

There is no single 'tournament standard'. You would need to check the rules for the event you are looking to enter.