I certainly could not get an ATA-6 drive to work in a 2400c recently, and had to downgrade to an older drive that I had to do some hunting to find (as it needed to be quiet). The 2400c and the 3400c are similar machines, so you might indeed have problems with a 3400c.
I gather, however, that others here have succeeded in getting ATA-6 drives to work in a 2400c, so it is probably just hit and miss. The good news is that the 3400c is relatively easy to work on, so you can experiment with a number of drives.
On your other question, I put a very cheap Chinese ATA-CF adapter in a 2300c with a 4GB CF card a couple of years ago and it worked perfectly first time without complaint. I'd say go for it with the Addonics adapter, which should be better quality, but the one thing I'd advise is to be careful to install a layer of electrical insulation between the adapter and the extensive copper shielding in the bottom of the machine. Otherwise you'll get yourself a short-out.
I gather, however, that others here have succeeded in getting ATA-6 drives to work in a 2400c, so it is probably just hit and miss. The good news is that the 3400c is relatively easy to work on, so you can experiment with a number of drives.
On your other question, I put a very cheap Chinese ATA-CF adapter in a 2300c with a 4GB CF card a couple of years ago and it worked perfectly first time without complaint. I'd say go for it with the Addonics adapter, which should be better quality, but the one thing I'd advise is to be careful to install a layer of electrical insulation between the adapter and the extensive copper shielding in the bottom of the machine. Otherwise you'll get yourself a short-out.