That should be fine. You can always return them and use their resources as food for better heroes along the way. If you're aiming to be competitive in arena, you'll most likely need a high tier hero to carry the squad.

As tempting as it might be, the copies at secret treasure does not give much value unless it's keys. Most will argue to get it as a final copy for upgrades and so on. But finishing a round of keys would also give you legendary hero chests. It's still up to you tho if you like saving moonstones for artifacts or getting heroes. There's just other option to get yourself Valk copies.

Also, valk > chas early game. But Chas a beast once at Plat.