So Pete, we don't know if Fredo was a capo. If he had men, then it seems likely. However, there are timeline or continuity problems.

In the GF, he is running a casino hotel and successfully. Apparently, he doesn't have any men at that point. Then by GFII he has men and runs a brothel or two and may have been a capo. Now, he can legitimately complain that he was stepped over for leadership of the Corleones. But, his complaint about only being sent hither and yon on mickey mouse assignments falls flat. He was given substantial responsibilities and a leadership position in the family.