If there was no application at all then the sequence was working perfeclty from sqlplus
There is no problem with the sequence, with or without the application.
Budget does not come in to play here. The application is already available. It need to be customized by including the functionality of having a auto number generation across multiple schemas of a database. And yes why not an application have more than 150 tables if there are 15 schemas involved. Its just an assumption and nothing else which can be clarified if @OP answers the question of letting know to how many tables are used in each schema by the application.
Budget is meant, if your database can't handle 150 triggers, then you should get a more powerful system.
The application should not be customized, the autonumbering should happen at the database side. Even if the database has a 1000+ tables.
Pilgrim.