Both Brown, a Broncos free agent, and Schulters, a 49ers FA, would be good fits in the Giants secondary and would also come much cheaper than Williams. At this moment, the Giants' only starting caliber safety is Omar Stoutmire and don't think the club would be all that comfortable asking him to play a full-time role.
As for Comella, the trip to Houston is his first free agent visit, as it appears more and more likely that the Giants don't intend to re-sign him.
Also, New York has shown some interest in bringing back DT Christian Peter. Peter bolted the Giants last year and signed with the Colts, but was released earlier this off-season. Re-signing Peter could free up the Giants to move Cornelius Griffin to defensive end, an idea that came about last season. Some in the organization would rather see Griffin at end than tackle.