My thing here is that the squirrels seem relatively easy to get access to. Their scroll is allegedly in a (very small) country in the woods somewhere. Once we get the dog scroll and can get the frogs to point out where Jiraiya had that fight at, we shouldn't have too hard a time finding them.
Weighing ease verses usefulness, I'd say that the Squirrel Scroll would be a nice and easy one to get, but I'm not sure that they could help us very much. Yeah, there's the prestige of another summoner in the clan, but that could be said of any of them. And the techniques that the Squirrel Clan might provide wouldn't really help Hazou --though they might help other clan members.
(Would Keiko even be allowed to use techniques provided by other summoning clans?) Not quite sure what material goods the Squirrels could provide that we, ourselves, don't have access to already. I'm pretty sure that they live in a forest biome and we live in the
Leaf. Do we know what the Squirrel Clan's reputation is on the Seventh Path? We might want to be wary of signing up with turns out to be a thieving gang.
Though, since the two biomes are rather similiar --if not exactly the same --an argument could be made that the Squirrel Clan was
made for Leaf. I mean, Team Uplift grew up in Mist and so are likely more at home on an aquatic battlefield than a forested one. I suppose that the Squirrel Clan could teach them how to move through dense forests properly. And, now that I think about it, that might even help our naturalization process in with Leaf, too. We wouldn't "move" like a ninja from Mist, anymore, we'd "move" like one of "them." It could help persuade the less diehard traditionalists, I suppose?
Anyhow, because of the above paragraph and the relative ease of acquisition, I admit that I could be persuaded if the Squirrels have more combat ability
(or a hidden ace) than they seem to. But right now, even with the difficulty, I'm still leaning towards the Porcupine and the Otters... though the Kraken might replace Otters for second place, since I can think of a few Lore-reasons that the Krakens would have Sealing capabilities.
Edit: I wonder if getting the Otter or Kraken Scroll would get us some negative PR from Leaf. Or it
could be a positive, stealing away Mist resources and making Leaf stronger. A very ninja thing to do.