1 - Fine.
2 - No opinion.
3 - Rather than getting into wacky ratios, I'd probably say something like:

Each side of the Landing Craft can carry up to 3 Rhino hulls or up to 2 Land Raider hulls.

You could therefore max out at: 3/3, 3/2, or 2/2, which is plenty of combinations, is logical, and matches the size of the model properly.

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Currently doing a plastic scenery kickstarter

1. Fine.
2. Fine
3. I find the mixing of chassis a bit odd but E&Cs explanation has some merit.

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1. Fine
2. Possibly OK, but it opens up a tortuous exception list (may transport twelve infantry except x, y, z)
3. I would prefer mixed hulls, and suggest the wording be "four Rhino or Land Raider variants (six Rhino variants if no Land Raiders are taken)"

1 and 2 seem fine.

I think E&C's suggestion of each side being able to carry 3 Rhinos OR 2 Land Raiders is best.

That's kinda my thought. It might be better done with new units "counting as xx for transport purposes", as we did with the Death Company in the Blood Angels list rather than modify the Thunderhawk and Landing Craft's datafax, for example.

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Currently doing a plastic scenery kickstarter

Are you OK with Support weapons being transported?

I guess there is no reason why not, but just making sure they are part of your design view as well.

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