Is it advisable to reload Pro Mini with the bootloader originally designed for UNO to save 1.5 KB flash space?
You can just flash the newest optiboot_atmega328-Mini.hex. That one should also be only .5k in size, as it starts writing at the same address (7E00). 

If you divide 7E00 by two you get 3F00 (This is because AVRs use 16-bit words for addressing while .hex files work with bytes (8-bit)). (I figured that one out, just now)

3F00 is exactly the start of the Boot Loader Flash Section when you  have selected a 256-words Boot Size (page 291 of the datasheet). 256-words is the 0.5kb you want.

Just make sure you have the BOOTSZ1, BOOTSZ0 fuses set to 1

As for the boards.txt. I think you only need to change mini328.upload.maximum_size=28672 to mini328.upload.maximum_size=32256.