Yup, full round action is the same as a full attack action timewise, it just eats up your full action (just one 5' step possible in addition to it). It does not take a full round to complete, like those summon spells.
About the metamagic, the signature spell works just like any other spontaneous casting, so you can also metamagic it just like anyone else (bards, clerics and sorcerers).
Bye
Thanee