Specifically I’d like to know about Roost and Call, as in what is their purpose?
Also does this take your companion’s spot? I.E. can you use the little greenish thing as well?
Thanks in advance, and sorry for making yet another questions thread I’m just waiting for that OBT announcement date which gives me too much free time. 
Roost and Call here for promote their cleaver ai.
Yes can have both bird and rat in the same time.
Before think about this kind of build i recommend to read LN like Only Sense Online first.
Although you did answer my second question, I’m still unclear on what the two skills do.
Also I’m not sure what Light Novels have to do with building a class in TOS. 
Not that I won’t consider reading it. 
your falcon gone after you use falcon skill so you need to call them back every time roost make them stay after skill use