I think J-1 would be the most legally correct (IANAL and all but I have some J-1 experience). YC should be able to bring founders on board. Since J-1 is for education and the program is basically an education.

J-1 is also quite openly (ab)used to bring employees over who do not qualify for a H1B or O-1.

Only important thing to remember is to pay the J-1 "student" otherwise they are (depending on origin country, but most I think) subject to a rule preventing said person from returning to the US within two years.

I think J-1's are 18 months max. Plenty of time to get a business up and running. By that time if the founder is successful (e.g. take on more investment) they could probably employ themselves with a H1B or E-2 ("investor") visa.

But I'm sure the J-1 has a bunch of issues concerning founding legal entities - this part I have no experience or knowledge of.