As stated, if you are going for two different types of RAM configuration dual channel won't be possible, thus lower performance The two slots must be exactly the same otherwise they will run with the speed of the lower specifications (or it might not even boot, which will mean you will have to manually set the same timings in the bios for it to post)
This is a tricky one, you get double the bandwidth in dual channel mode, but you do lose 1GB of memory, since you stated you have 8GB I would recommend getting one 4GB stick, save up some money and buy another identical stick.
Disregarding the limitation, more ram is better almost* all the time, so go big.
2x2>1x4, 4+1>2x2 but 4x4>1x8
*It will depend on what you are using it for.