Cheaper and easier to buy a B&W hitch and leave the trailers alone. Granted having trailers with the same size hitch is more convenient and replacing a rusty one (depending on how bad) is important.

I run this one, never had a freeze in the winter, always lets me spin and adjust for whatever trailer I'm hauling which is one of 4 trailers, different heights, 2 x 2" hitches and 2 x 2-5/16" hitches.

My snowmobile trailer is aluminum and has some sort of cast steel hitch on it, doesn't seem to be a problem. I don't see any corrosion radiating away from the edges. Probably a decent idea to treat it with some sort of never seize or research a product that might be specific to what you are doing.

 

My Aluminum two place Triton has had a painted steel hitch on it for going on 30 years. There is nothing between it and the aluminum that I know of. No sign of any issues.

 

I would suggest putting an anti-sieze compound between steel hitch and aluminum tongue, and around the steel bolts, for best protection, if you go that route. And use it in the receiver and around the hitch pin on the truck to make hitch changes easier.

I use a hitch like shown in post #2 and it makes life easy. But they are not cheap. Apply something like WD40 2x per years to all the pins, etc.

 

Just get a 2-5/16 trailer coupler unless you want to fight with a vehicle side ball in the cold or warm when it's rusted beyond coming out of the receiver. Use what you want to shim need be, if you buy from a trailer place they should be able to get you a direct fit size
You can get a multi hitch, hopefully the six year old welding it was on top of their game that day...

 

I really, really like the Demco trailer couplers. So much easier as they "snap" on the ball.