No, you can't - Flash Pool is not supported on FAS3140.
Flash Cache support is limited to existing installations only & (max 0.5TB per HA pair), but (funnily enough) - it is not supported at all on your ONTAP version. You should consider upgrading ONTAP to 8.1.2 as quickly as possible.
yes, true - we always recommend the latest P version available.
It tends to be shooting to a moving target though: at the time of writing we are on P4, but who knows what version will be available in, say, one month? (by that time 8.1.3 may become GA, or 8.2 will be more feasible to implement if SnapDrive / SnapManager gaps are plugged)