Ideally in a raid setup you want the drives to be the same make and model.
You can mix them but I wouldn't. I would keep all drives the same rpm though. I don't think buffer size will affect it nor SATA version.
Just make sure you keep regular backups of your data.
This gets pretty iffy(?) real quick because you have drivers trying to optimize access and the firmware on the drive trying to optimize access times.
1. You can mix drives if you want - the drive reads the full track into its buffer and transfers it to the system - once done it says complete. It may be slower to mix drives or it may not be - I havent seen a benchmark on it yet. Because of the complexities involved we all go for the keep it simple approach - and I recommend you do that.
2. Same difference
3. Seek time is the amount of time to reposition the heads. Avg access time is seek time plus 1/3 of the rotation time (the manufacturers have used different ratios here).
If thats all you have - put them together and see what it does. I am a fan of experimenting.
Do keep a backup. While easy to go into a RAID configuration, its not easy to get out of it.