Don't Panic!

Do the Bucket test for metals to verify metal in the water. What does the inside of the toilet tank look like?

Bioguard's Metal Magnet Plus is (according to MSDS) 36% HEDP and 1.8% Phosphorous acid. HEDP is a recommended sequestrant and should help get metal in suspension. I'd let 'em put it, hopefully it will clear the water.

You may want to put trichlor tabs in the skimmer - couple reasons. 1) You need chlorine and CYA in the new water. 2) The tabs dissolving in the skimmer will raise chlorine and lower pH in the water before the filter encouraging the metals to drop out of suspension onto the filter. You can do this because you have a sand filter and run the pump 24/7. If you do choose this startup, don't let them put the dichlor (Super Soluble) in the pool.

It may also be a bad idea to put a lot of chlorine (Burn Outs) in the pool if there are metals in the water - it could cause metal staining on your new liner.

The Algae All 60 is Polyquat 60% (from MSDS) which is the only algaecide recommended here. It will help keep algae from getting a foothold while you're starting up. Especially a low chlorine start you'll need if you do have metals.

Please consider any warranty requirements before deviating from their plan.

Take a full set of readings (except CYA) on your new fill water and let us know.

Congrats on the new liner - Good Luck.