Is it possible to have a non-integer multiple of minimum length in MOSFET technology?
It's always been fine for technologies I've used to have a length just slightly greater than the minimum length; it does not need to be an even multiple. 

I wouldn't count on being able to have the width be 0.25 μm, it is likely some greater value.

You will need to get your hands on the design rules for your process. They should be automatically checked by DRC.

There are reasons to use multiple copies of a transistor in parallel instead of just increasing the W/L ratio. For example, if you need one transistor with 5 times the W/L of another transistor, it's better to use 5 copies in parallel for the second one. But it's not required, just a strategy for better matching.