You do not have to slab the planet to get the full income from the 100 hotels. Its based on planet size. In your case you could build all 100 im afew cities and as long as you do not build any additional hotels in additional cities, you will not damage the income
Mnsf is determined by the number of slabs per planet so if you have fewer slabs you must also have fewer of that facility type to avoid losing income. For example if you can have 4 hotels per slab on the planet it doesn't matter on which slabs you build them but it does matter how many slabs you have overall.
Syn is correct unless they changed the code recently. The MNSF is per slab on the planet, regardless of the size of a planet. So a single slab on a 20x20 will only allow 4 hotels before you start losing income. 10 slabs and you can get up to 40 hotels before you start losing income, etc.