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.
Flash Pool is supported on 3200 (excluding 3210) and 6200 filers as well - the problem of 3140 is low memory: only 4GB, versus 6GB on current 2200 series systems.
Your current setup with Flash Cache is definitely unsupported & from the top of my head there is a good reason for that: a bug which may cause out of memory condition & filer panic.
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)