Definitely wait until you're done rolling before making the trade. I haven't seen Yama do much and it's playstyle isn't too different from existing leads.
I used the stones earned from the quests and the free dailies to pull MH again and got Nergigante so that's cool. Zinogre is one of three fujin-likes in the game I'm missing which makes him an interesting bit of trade bait. Here's a list that I can think of, if there is some NA collab or REM monster I missed let me know:
Fujin
Uruka (DBDC)
Lugh
Gilgamesh
Akuma (SF)
Hiko (Kenshin)
Leona (KOF)
Qilin
Panera
Borma
Reeche
Reeche-Lite
Missing:
Dark Raizer (not in NA yet but I'd be surprised if we don't get him)
Uesugi Kenshin
Zinogre (MH)
After having messed with him a bit, I really like Zinogre. As a sub, he has great stats, lots of TE and OK-ish damage. He also circumvents the Uruka problem with skill-charge. I even have found him to be kind of fun to mess around with Sensui as a lead. I dont' know if he is worth a trade exactly, but I feel like he is one of the stronger options. I really wish I had a second one, as he is also a solid equip.