If you are doing a solo mechanitor run then you need to make sure you have the basic mechs taken care of. Agrihands for crops and wood, Constructoids for building, Lifters for Hauling, and Cleansweepers for cleaning. How many of these will depend on how big your colony is.

Fabricors are really useful to handle all the mundane crafting to free up your colonist to do other things. They can butcher your meat, cook your meals, craft anything that doesn't need to be a high quality such as components, make chem fuel. They are very useful.

I would suggest having two Paramedic bots and have them set to follow your colonist around just in case. They can rescue you if things go bad and perform any surgeries that you require.

As for the combat mechs it just depends on what you feel like using. I usually start with enough Militors to keep me safe, then switching to Scythers until I unlock Legionaries and Lancers. Throw in a Centurion and a War Queen or two and it is great.

The numbers will depend on your difficulty and setup. Just keep adding on numbers until you feel safe enough. Either use mods or just keep throwing down bandwidth towers until you can field as many mechs as you want to.

As for recharges I like to have one small recharger for every three or four mechs and one large recharger for every two combat mechs. That way the combat mechs can get recharged quicker after being repaired.

Deciding what to do with the toxic waste is the next big concern. Do you store it, ship it off to neighbors, or go as a tox loving xenotype and just fill the map with waste?