The first issue would be the sizes and abilities. Like characters like Master Mummy, Max Brass and Mechanica can't really fit with their body sizes. And Byte would be nothing like himself in ARMS without Barq. The 2nd issue is that most of the ARMS fighters have different ARMS in general. Like if you wanted an Echo Fighter for Min-Min, Ninjara is probably the one who's ARMS are most similar to Min-Min's ARMS.
-Buff can function like Megawatt with a few differences, being that Buff is faster than Megawatt, less damage and impact, and has added homing.
-Chakram can function like Ramram.
-Triblast could do something similar to Dragon in which they are ranged weapons, but Triblast shoots out rockets in a 3-way shot, but each individual rocket deals less damage and can be reflected.
Not to mention Ninjara isn't too far off from Min-Min's height. He's taller, but that could be compensated by making Ninjara bend over a bit more in his idle stance.