Having the shrimp there shouldn't bother a small pleco in terms of space.
If you want snails though, I'd go with ramshorns. They'd be easier for an assassin snail to keep up with, plus they're cute. The betta will also likely eat a few of the smaller ones and if you don't over feed they won't explode in population.
thanks ramshorns sound like they'd fit the bill perfectly.