Having a ppm meter allows you to assess your water and how much stuff in it, it also allows you to understand how adding nutrients to that water raises the ppms.
Like watching what goes in is important, watching what comes out is important too. It maybe different for soil, but in coco, I try not to let the ppms go over 1500, 2000max, for my runoff - 1000 being my sweet spot. In my first grow, I had ppms in excess of 3500, 7500 in a few cases, and my plants were really messed up. You don’t necessarily see the impact in vegetative stage, but once the plant starts flowering, the impact of an unhealthy medium from a pH and ppm standpoint will really stand out in the plants leaf health.
Be sure to get PH up and down and some Standard Reference Solution to make sure your meter is reading accurately. Do not get a soil probe type as they are worthless unless you spend a lot of money on one. Separate TDS (ppm) meter is wise.
No, I would wait until they outgrow the pot they are in (one to two weeks) then to their final home. The size of the final pot is personal choice, taking into consideration how large your growing space is, how many plants in that space, and how large you want your plants to be.
I normally start in solo cups for the first 1.5 – 2 weeks, then to one gallon pots for about one week, and then to the final pot, which for me is 10 gallon fabric or previously 10 gallon nursery pots.
This is based on have an 8 X 8 X 9 foot room and I like to grow a few large plants rather than a bunch of smaller plants. That is purely personal preference.