Yes assassins will breed in freshwater just not as rapidly as pond snails or ramshorns, but seeing that they roughly eat a snail or so a day they are a pretty good way to keep an overall snail population in line.
I have my ramshorns in seperate tanks though, since I am going for different color variants of ramshorns. I usually just feed the assassins the normal brownish ramshorns and pond snails. I either net or physically pick out snails from other tanks. If you have ever bought live plants than you will most likely have pond snails, I have them in 5 of my 7 tanks, the two that do not have them do not have plants yet.
What are you trying to do, do you just want assassins or do you want to have an assortment of snails? Ramshorns, pond snails, mts and assassins will breed in fw
the only caveat is that the assassin is the only snail that actually requires a mate, so you will need to get a few I'd say about 4 or 5 should be good if you want them to breed since they need a male and female to mate and as far as I know there is no readily available method to sex them. The other snails mentioned are hermaphroditic and can reproduce by them selves or with a mate snail as they can pick and choose if they want to be the guy or girl with each breeding.