You can use Pie-Register regardless of users count. There is not limitation on number of users in your WordPress. with Pie-Register you can enable Admin Moderation or Email Verification to stop spammers. When you have Email Verification ON and you set the grace period, all the unverified accounts will be deleted when it meets the time limit. Regarding your other question, I am afraid you can not send existence users a verification email with PR, however, you can have a custom code written to accomplish this, you may want to get a quote on this, please use our quote form on our website.
Please feel free to let us know if you have further questions.