there are two breeds of senche, Regular and Raht. Ones bipedal, ones quadraqed.
No, both Senche forms are quadrupedal... One's just bigger and meaner than the other.
Also, to expand on my case for playing as a Senche-Raht:
You say it would be like playing a horse, but if so, you are either ignoring established lore on the Senche breeds or trying to create an arbitrary balancing act. Though Senche are used as mounts, they are just as intelligent as any other khajiit, and are individuals in their own right. In fact, it is possible that a Senche-Raht could even be a battlefield commander.
Though the lore is mostly silent on whether the quadrupedal khajiit can speak (all we know is the Alfiq can't), nothing says they can't either. The only times they appear in lore are a Dance in Fire, where
none of the khajiit speak, and in the interviews with Cherim and Jobasha, where they are only described in a general manner. I personally believe that only the Alfiq and possibly Alfiq-raht cannot speak, simply because they are too small. The khajiit appearance in Morrowind already prove the "beast head" forms of khajiit can speak, so its not too extreme to apply that to the larger quadrupeds.
It is also known the Alfiq are at least competant spellcasters, as demonstrated by the book
Mixed Unit Tactics because although the author believes it was the Dagi casting the spells, he made it clear he was ignorant of the nature of the Alfiq (going so far as to call them "housecats")
If this is a war-style RP, the unbalancing power of the Senche-Raht is negated by the akaviri supersoldiers we are going up against. Also, though a Senche-raht may be unequalled in melee combat, you have to remember they are bullet-magnets, cannot use weapons, just as vulnerable as any other living thing to critical strikes, have limited mobility (as agile as they are, their size makes it difficult to dodge attacks), can't use weapons (Major drawback), and pretty much can't fill any role other than heavy assault. They are sort of like Hunters from Halo (If I remember, the hunters were usually easier to take out than most Elites)
And I find saying the Khajiit and Ka po'tun will ally simply because of their physical similarities is empircally proven untrue, as they have far more in common with the Bosmer than any other race, and you can see what happens there. The Ka Po'Tun are closer to the Altmer than any other Tamrielic race. Also, another reason the khajiit and Ka Po'tun will not ally: Game balance. The martial prowess of the Ka Po'Tun and versatility of the Khajiit will make them the greatest fighting force on Nirn.