You can never boot GB or GBC games on a GBA flashcart.
Even though the GBA is backward compatible, the cartridge are not.
GB/GBC cartridge run at a different voltage, and when inserted in a GBA they trigger the different hardware mode to run those games.
I have a Flash2Advance pro, but the Sram is brocken, therefore I can't use it with regular GBA games that need to save to the battery.
Therefore I bought a GB Bridge, which is a sort of U-shaped converter that you can connect your flashcart to and slide it into the GBA, changing the voltage of the cart making it boot in GB mode.
I put all my GBC games on it(included Cannon Fodder) and they all work flowlessy as on teh real hardware.
However they never looked so good in the past, now that I have a backlite GBA SP.
Most GBC games don't even use a battery to save, they give you passwords, so I'm fine.
The GB bridge I believeonly works with F2A and the old vision-something card that are no longer in the market.
But if you could get this kind of equipment than you won't have to pray for an emulator anymore, but you will get the real thing!
Another cheap solution would be to buy real used GBC games, there were quite a few good ones, but not that many after all, and they must be really cheap these days, plus they look magnificent on the SP2 !