Yes, all field passives work. It’s just entrance passives which do not.
Slayerbane all damage is low. Excessive force (and all piercing) ignores enemy defence so it increases damage by a lot. Even on final form you actually need 2 enemies with kills before it can one-shot them (it does about 2800 damage * number of enemies with kills). In second form the damage is not good enough.
Team cost stops increasing past HR 120, which honestly you’ll reach before too long (takes about 3 months?). You can see a table with all the details here: KD's Hero Rank guide
So basically as you rank up you can fit more into your team and just figure it out along the way. When at max cost you’ll typically have 14-15 “real” monsters and 1-2 low cost ones or do some balancing with secret skills or using super epics to fit things in.
Poseidon+Myst is a very strong combo that you can make good use of. Dusicyon is a more flexible and powerful tool overall. Honestly, it’s a decision you have to make yourself about what you want to go for. If you decide Poseidon+Myst I recommend you wait until you have all the required tickets before spending any, in case by that point you’ve actually changed your mind because your situation has changed.
Absolutely! The damage is insane when one of the enemies is asleep. It allows instant charging of bloodthirst but can also be great to kill HG enemies, remove shields or heal Mantisamurai off HG (even if it’s non-critical this can cause the opponent to do an extra attack to kill it). The fact it’s instant is what makes it especially strong and a clear choice to turn on. For reference, I’m currently using Manti in the FL in bug union and the first setup I built of the team didn’t have the SS because I didn’t think I’d need it (and I wanted cost for elsewhere) but when playing the team there were so many instances I wished it was on. Since turning it on the team plays a lot better.
Feel free to ask more questions 