They come with Teflon tape, would you use 3M 4200 as well?
No. Just teflon tape, NO other sealant
Be careful not to OVERtighten. Tank fittings are NPT standard, which means they're slightly tapered--too slightly to be visible in such a short distance...over-tightening will put stress on the "collar" that will cause it to crack...maybe not immediately, but soon. So no more than one turn past hand tight plus enough to aim a 90 or 45 where it needs to go...they're self-sealing, so that's all you need. And use only plastic fittings in a plastic tank.
Low in the bilge
will make it harder to vent to keep it aerobic...