SSHD vs SSD vs fusion drive.HDD on PATA case? RAM maximum storage.

What I want to do with my Imac (model 7.1/ EMC 2133) is:

1. Installing an SSHD which I read is the best deal regarding this model vs installing an SSD on the previous HDD spot while replacing the optical drive with the old HDD and creating a fusion drive.

Is this doable to create a fusion drive because I read that there should be issues?

Also, how can I insert the 3.5'' HDD (old internal drive) on the optical drive position as to the best of my knowledge the dimensions are different?

2. The sad part is that the PATA drive is really slow (100mb/s) and except of the fact that the dimensions are different(which part I ask above).

Would it be possible to use the second drive as a backup device for Clone backups or with Time Machine or it has to be an external drive whatsoever?

Would the time duration of the backup be ridiculously slow as well on the PATA spot?

and

3. I also read on the net that some users upgrade their RAM to 8GB.

Can the hardware restrictions be bypassed and install even more Gbs?

It would be nice if you include answers with tutorial and references.

Thank you all for your valuable time!

Cheers,

Theo