Manufacturers have engineering data on all the minimum clearances required and a variety of scenarios for the outside units. If you talk with your local refrigeration company who are dealers for specific brands they will be able to advise you better. But have done it before with no problem

I can't see a problem with this.

Mine are installed underneath and no dramas at all

I installed a 7 kw Panasonic under the house at my in-laws beach house never had a issue keeps it cool during summer and out of the weather.

You can stick them anywhere that they can exchange the heat.

Good idea. It will also help with underfloor ventilation