There's no way tha a ten gallon is enough room for more than one hatchling. A ten isn't really good for much anywa as its very hard to keep a thermal gradient in such a small space. A 50 is a decent size for the three or four hatchlings. You could safely keep three in a thirty long though i think.
Also careful with plecos.
Firstly they are not from the same part of the owrld as snapper so who knows what foreign orgainism they may be harboring that may adversely affect the turtle
Secondly they make big poo..they are messy fish and usually make more dirt than they seem to consume (never and answer to a dirty tank)!
Third they grow fast and get huge. A two inch pleco if properly fed will be 8 - 10 inches long inside of a year.
Fourthly, There is at least one instance of a pleco decapitating a turtle and another in which a pleco attached it self to the "face" oif a larger turtle and did some damage to its eyes.
Lastly, its usually ther other way around. As you suggested, the turtle may eat the pleco (again see the first remark)