Say I have a simiyiar with capacity 8 and a eddicus with a capacity of 15.

Can I load the eddicus with 15 troops then load it into the simiyiar even though it can only carry 8?

Yes, so long as you don't try to offload the passengers from the Eddicus onto the Simiyiar once it's docked.

Same works with items, you can put small items in a medkit, which can go in a belt which can go in a backpack

Awesome thanks very much for the help :)