Yeah, currently I think they are the "strongest" pair at a score of 68.
Ryoga + Kazuya = 125

Sanada + Yuki = 73.
Kirihara + Renji = 68
Jackal + Bunta = 50
Golden Pair = 42
Silver Pair = 40
Jimmies = 40
Just to put things in perspective.
Perhaps for season 3 we make negative side effects if certain people play in singles. Or just make Kirihara + Renji get a +10 like pretty much everyone else for simplicity sake but I agree with you Chaos I think everything is fine for Season 2.
You always forget my Gay Pair! It's 36.5 , but I always say they should be 41.5, adding both Hitouji and Koharu +5.
Well, everyone can play doubles with everyone. The difference are the bonus of the pairs for example:
first level being Fuji-Kawamira, Momo-Kaidoh,Oshitari-Cancer,Rin-Chinen, Ishida-Kenya,Akazawa-Kaneda,Momo-Kawamura,Amane-Kurobane,Saeki-Itsuki...well, regular doubles pairs that we have seen in series.
second level with Gay Pair, Jimmies, Kenya-Gin,Inui-Kaidoh
third level with Rikkais pairs and Silver Pair
and
the biggest bonus for Golden Pair, of course.
And Feats for Doubles Experts (when this players play with different people than their usual pairs,and only two in each school) and Doubles Cancers:
Doubles Expert: well, Oishi,Kikumaru,Ishida,Kaneda,Minami, Higashikata,Ootori,Choutaroh,Rin,Chinen,Itsuki,Saeki,Jackal,Bunta,Rin,Chinen,Hitouji,Koharu
Doubles Cancers Ryoma, Tezuka and Chitose (because of Pinnacle of Wisdom),Kintaroh...And more could be added, like Akutsu or Tachibana (I an not imagine them in doubles)