RID, KNN and PON are typically the "rarest" breeds. Or at least they're still working on establishing a presence on the breeding leaderboards for the most part. Each have their own challenges.
KNN are dependent on genes, so it makes them a bit of a challenge to breed, as KNN x KNN matches can end up as SH, if the Lp gene wasn't passed along.
RID are just typically known as a weaker breed, and they're good if you're interested in developing breeding stock to get them a bit higher up on the leaderboards in the long term.
As for PON, breeding them are height dependent. I don't know the exact height, but it also introduces some RF when taller PON x PON matches end up as SH.
But if you're up for a challenge, they're pretty fun i've heard! AA and WB lines can always use some strengthing too ^^
As for breeding goals, i typically set myself small checkpoints. For AD breeding, work towards breeding an EEE. Then a EEW, then think about using items in your matches, and set financial checkpoints. (save 50k, save 100k etc etc.) Sometimes it's nice to write these goals down in your bio, then strike through them once you've accomplished them!