skinny on monty rim?

can you mount a thinner tire onto a monty?

so you could have a trials tire, then an indoor tire?

Is that even neccessary?

nope, the mod rim is an inch or two smaller than a normal 20".

not unless you can find a 19" freestyle tyre.
the monty/onza rims are a bit smaller in diameter to a normal 20" tyre.

and it’d be easier to get a new wheel for freestyle and change that over instead, esp since you can get a light duty 20" wheel for cheap these days.