"Copying" third party databases into central mySQL and keeping mySQL up to date

I have a question which is what I believe to be quite complicated and would like to see what experts like yourselves think about my solution.

Problem:

I am collecting data from a number of third parties into a central mySQL database. The third parties can have any database product (mySQL, Oracle, postgreSQL, etc). I will need to perform two tasks, which are the following:

1. On first start, replicate the full database onto the central mySQL database
2. Incrementally update the central mySQL database as changes are done to the third party database

Proposed solution:

1. I intend on creating database crawlers in C# per third party database. So say for example one of the third parties has an oracle database. I intend on creating a class which will query an oracle database for its definition and data and programmatically create sql queries to be executed by the central mySQL database. ex: create table XYZ (id1 int, id2 int), Insert into XYZ values(1,2), etc.
2. Here is the problem which is giving me the greatest grief. Id like to know if anyone has ever queried a database to effectively ask it "what are your latest changes", or what are your changes since dd-mm-yyyy hh:MM:ss. By changes I mean both data changes and data definition changes. ex: add new column, update a [row][column] value, insert new row, etc.