On Joshua,
He sucks. I think he should be even lower than he is.
On Ewan vs Rennac,
With the 10/5 rule, as opposed to 10/1, Ewan is greatly improved. Like PKL said, he can get the energy ring by himself which is important for freeing up your other units. Ewan has 1-2 range, and can go Summoner for Phantoms which is honestly better than mage knight.
Rennac's item finding is free, He really should be focing on getting items in Chp 15 so having him fight is actually a negative, Either promotion path you choose for Ewan, Rennac's range of attack is lower.
Rennac is just a promoted Marissa who comes later and doesn't cost any turns. 10/5 Ewan is "Knoll 2.0". He make all the same contributions that Knoll does + gets us the very important energy ring.
Ewan >> Rennac
Oh, and he should be above Knoll, obviously.