Yeah Sacro got it right, if you want say to run your kabalites as <Flayed Skull>, you cannot have any coven cult units at all in that detachment. If you say throw a talos into your flayed skull detachment, you lose all the obsession traits, artifacts and stratagems. The solution for that is running multiple detachments, say a kabal battalion, a cult outrider and a coven spearhead. I pretty much never run just as Kabal, cult, or coven army, I usually mix kabal with one of the other two, or all three together, but you can run things pretty much any way you want.
As for running characters, yeah it's kinda an annoyance that we don't have say a twelve capacity transport, but no way around that. My most common practice is running a small character escort in a raider, say 5 incubi, some archon's court models or a few wyches, and then throw in an archon and succubus there. The venom option isn't also as bad as you think, I've done it a few times, just an archon and a court model or two in a venom and my opponent rarely focuses it down because he's too busy dealing with more pressing threats like ravagers and reavers.
I think it's also worth mentioning that not all characters need to be in transports. Heamonculus in particular is usually good on foot because he will go the same speed as taloses (or talosi?), and give them his nice buff. I also usually run my second archon from my battalion on foot with my ravagers, one archon as a melee beatstick up close in the fight is enough.
The Night is Dark and Full of Terrors...