Yep, as long as there is enough floor space and hiding places for them both
I have always just put them in as you would any tropical fish, equalise the temps and top up the container they are in with tank water over 15 - 30 min.
If you have an established tank with fish in already it does pay to quarantine new arrivals for a few weeks, maybe with upto 6g salt / litre. New native fish often will carry whitespot
With smelt you want to get them into cold, high O2 water as soon as you can to minimise deaths.