I totally agree with you; Toph and Korra seem they'd be easily friends (and might still be, if Toph has not died as of the beginning of this series. And admit it; it's possible. She's younger than Katara, and Katara's still alive, so unless they outright say Toph died...)
As for Lin, well, my guess is the two will get along later in the series; or at least she'll learn how to tolerate her.
I mean, srsly, who else (of the currently provided cast) has the potential to teach Korra Metalbending?
Maybe not now, but possibly toward the end of the series? I could definitely see her begrudgingly learning to respect Korra, perhaps teaming up with her (or at least somewhat appreciating her help), or something else of that order. Seems kinda like it's necessary, as a matter of fact.
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)