i had one tie rod end replaced because it was in really bad shape, and it was before i found the heavy duty ones from ecs...would it be a bad idea to put a heavy duty on one side and an OE on the other? i'm thinking it's ok because i don't think the heavy duty one is any stiffer, it's just stronger...i don't know though

 

Stronger is stiffer and probably heavier.
Always a good idea to never mix parts.

 

I can't imagine you would have problems with an oem on one side and the ecs ones on the other.