Judging by my limited experience, you should have air dominance against Kouma, and you should try to capitalize on it. When you are at a safe distance, set up a black cat to try and cover the ground. Tin cat is stronger against other chars, like Nero and Wara.
As for combos, here are some off the top of my head:
2a5c(4 hits)2b4c jbc jbc AT (basic BnB)
2a5c(5)623b 5c(5) jbc jbc AT (corner combo)
2a5c(4-5)4b 236c BE236a 5c (4-5) jbc jbc AT (100 meter combo for damage, should be getting 4.5k and above)
2a5c(4-5)4b high 236c 214b (black cat) 5a5c(1-2) jbc jbc AT (throw them on the cat for knockdown)
Also you can end a midscreen 236c combo with 2b for knockdown, and try dodge mixups or telport mixups (teleport behind them, fake jump 2b, etc.).
After you end a combo in the corner with airthrow, you can easily punish all 3 options. When you get your opponent to respect you, start throwing tin cats on them. You can throw them between the hits or go for a fuzzy:
deep jc -> 2b4c aircombo / jc j2c land 5a5c combo
j236c is good for range punish, but C-Kouma doesn't have many things to punish except for heat activation. I don't think the orb special has any good uses. Overall H plays pretty much the same with the added bonus of 5a6aa for better blockstrings, her 5b is better (hits low) and has h-moon bunker, which is one of the few ways Len can get out of pressure.