1) If I cache a object in KEEP pool, does Oracle prefers FULL table scans on that table?

No. The access path for a table is determined by its availble statistics, and expected number of rows from the table, the number of blocks to read and the CPU cycles.

2) I wrote the following query to Identify the objects in Buffer Cache. Is it the perfect way to get the list of candidates for KEEP Pool.

Since KEEP pool is always going to be smaller than DEFAULT pool, you may prefer to keep small reference tables and their indexes.

Once Tom Kyte suggested not use KEEP pool at all. Define the buffer cache only DEFAULT pool, and let oracle manage it efficiently. Some of us may agree his point. I leave it to you.

If you get good performance after defining the KEEP pool, then go for it.