Since I have a whole kettle full of adopted agents, I think I can partially answer this by referencing the types of agents I currently have.
1/ The ones I created. These include people like Dafydd, Narto and Lou, and a handful of others. Obviously there's no problems there.
2/ The ones I co-created. Most of the cast of The Reorganisation were co-created by myself and Vemi. Again, I don't see any issues here.
3/ Agents who have specifically been given to me by their author. This is mainly for the Building Maintenance crew, who were passed on to me by JulyFlame when she left. Again, no problems.
4/ Agents who my agents have been on co-written missions with. This covers primarily Vemi and Constance. The latter is something of a special case halfway between this and 3. I only do this when their author has left, and there's something specific to do with them (Vemi's role in Crashing Down was planned before Vemi left, and Constance is of course married to Dafydd). Selene is also sort of in here, but she's also partly 2.
5/ Associates of those in 4. This is how I got Penny and Steve. I consider this reasonable, since they only really exist as appendages to the 'primary' agents.
6/ Those whose authors I worked closely with. This is essentially the reason behind my adoption of Starwind and Estelnar; I worked a lot with Boarder!Starwind, on the Multiverse Monitor and some of my stories, and occasionally have roles which her agents fit very well in.
7/ Long-abandoned agents. By this I mean really long, and I lay no claims of exclusivity. This is more of an 'unauthorised borrowing' style of thing, and lies behind my occasional use of Architeuthis, Black and Irvine, and very rarely Jay and Acacia. I justify this (to myself, at least!) by being very careful not to say only I can write them - and I know Archi has shown up recently in some DoI fics, so I apparently pulled it off. Basically, this is the 'rescuing and reintegrating into the PPC mythos' category.
There is another type, which I think is what you remember seeing recently:
8/ When an author has stopped writing their agents but is still around. In this situation, if there's a reason you want that particular agent (such as, recently, Lily wanting to use two Time Lords of Tawaki's) you can just ask the author if you can adopt them. The worst they can do is say no!
Obviously, the foundation of some of my adoptions is distinctly shaky. I do my best to keep the agents in character, and use them only rarely (Starwind and Estelnar, for instance, have shown up only once, in a 'but it's simply perfect!' moment). If their authors come back, I'll give them up without a fight - but at the moment, I think they're better off being used.
I wonder if that answered your question.
hS
PS: Out of sheer curiousity, which agents are you looking at?