How should I water an SLH Auto plant?

same way you water every plant, ever... saturate, wait for dryback, repeat. If soilless, 10% runoff, religiously. As far as fertilizer it's more about the ratio and overall concentration than the brand name. start with instructions and adjust based on what you see happening with the plant to avoid deficiencies/toxicities in future. Better products should not require a lot of adjustments. I'd recommend copying the ppms of Jacks 3-2-1 to start. proof is in the pudding, if their instructions result in nonstop problems, their instructions and products suck big fat dicks.