I think being careful not to ask for things you don't know about is good advice. Demanding that the system use 'Rails' when you can't program is probably not a good idea. Maybe your developer will choose it, but they'll do so for (hopefully) a good reason.
To that end, you probably shouldn't take on a cofounder that can't handle at least 2 languages. How can you make decisions if you have no choices?
Many times technical cofounders are used to basically "develop" the founder's idea. Anybody looking for a technical cofounder should give more than just ideas. Ideas are just 1% of the total work.