Hey there.
Welcome back. My advice: Go to the army list section of the boards. Dozens of folks just like me have written pages and pages of advice on various army lists, including shooty foot-slogging horde armies.
Few things for your specific questions:
1.
KFF gives a 5+ cover save for units on the ground. Not worth it; you can drop guys into ruins/forest/anything anyway and get a 4+ cover save.
2. Lootas should either be in previously mentioned cover for a 4+ save, or in a battlewagon enjoying AV14 protection. For maximum effectiveness, 3 squads of lootas, in 3 battlewagons, with a mek+
KFF giving the battlewagons 4+ cover saves will give you a good base of an army.
3.
SAG fits well into a shooty army. Attach him to Lootas and stick him in a battlewagon.
4. Aside from your core shooting base of lootas and battlewagons, you just start plopping in 30 man shoota boy squads with a nob and a powerklaw.
5. Want to keep your foot-slogging shoota boyz alive? Take a squad of 29 gretchin + 2 runtherders along, and spread them along your front lines in front of your advancing shoota boyz. They'll grant 4+ cover to the rest of your foot slogging army.