People will come to their own decision as to whether it is ok for feminists to ally with the alt-right when there is a common goal or enemy.
People have a variety of ways they make decisions about who to ally themselves with. It's not particularly simple in reality to do so, you can find links everywhere and people have such vastly different reasons at times for believing the same things. For many they make it simple, if the group is behaving legally and without violence they may collaborate on certain issues. It doesn't imply any larger agreement with them, and if others imagine it does, they are objectively wrong.
But I think you are mistaken to use the word ally in this instance and that's the problem. Some posters are making this assumption that everyone realises that having some kind of media interaction or public discourse is a type of allying, and we all know it but just don't care or at least think on balance its worth it.
But a lot of people, myself included, do not consider those things to have anything to do with being an ally. As a matter of principle, I will talk to pretty much everyone who is not directly at the time abusing me or threatening my person, even if they think my own rights should be taken away, people like me should be killed at birth, or they believe I should live in some sort of separate ethno-state. I think it might be even more important to have this kind of discussion with people who are not allies, who are your enemies in an ideological sense.
I think that sort of ability to talk about very different, opposed, ideas is the foundation of a civilised society and a democratic society, and one where people have the ability to think through ideas and hold the beliefs that make sense to them. I think that's more foundational than feminism too - you can't have a feminist discourse without that kind of society, so I am not going to repudiate that in the name of feminism or any other set of intellectual propositions.
As for giving someone a bit of youtube income - yes, it is true that every sort of media that sends some funding into private hands will in some sense support people who I may think are wrong. That happens whatever news sources I use and frankly I am a heck of a lot more worried about the financial power of the giant media conglomerates than I am of some youtuber. Yes, he will be able to talk about his ideas as a result of making some money, and in the spirit of public discourse it's my responsibility to make a better argument. Maybe I could have my own youtube channel, and people could fund me that way. But it's the giant media conglomerates that are actually able to control a lot of the public discussion and use their financial power to shut down views they don't like and promote those they support, in a way that is wholly detrimental to democratic free speech ideals. I live with that, still read those publications, while opposing that kind of monopoly and corporate power, so I am hardly going to refuse to ever listen to an individual on youtube that poses far less risk.