Alnico is not toxic (aluminium, nickel and cobalt), another thing is the fine dust if you grind the magnets. But if you grind it, it will mostly stick in the magnets anyway
Alnico is somewhat "obsolete-ish", although they can machine somewhat, with difficulties. It's hard and brittle, diamond saws and disc grinders would work. About 800 ºC, Alnico loses magnetism, so cool frequently!
Samarium and neodymium are way far powerful magnets than Alnico. But they are far worse to cut or grind. The samarium/neodymium magnet "blocks" are encapsulated, usually with nickel, and you should not peel their nickel sheath. Neodymium metal dust is a combustion and explosion hazard. Neodymium dust and salts are very irritating to the eyes and mucous membranes, and moderately irritating to skin. Neodymium quickly oxidizes in ordinary air. And neodymium magnets lose their magnetism in only 300-350 ºC. Powerful, but...
Same thing for samarium. Fender guitars were samarium magnets, just about the same as (powerful) neodymium magnets, and they lose magnetism about 700 ºC, way better than neodymium. But, if you peel off the nickel sheath, samarium slowly oxidizes at room temperature, and spontaneously ignites at 150 °C...
All in all, it might be a wiser move to buy 2 guitar pickups, and maybe cannibalize their pole magnets. They're next to nothing in China or Korea. You'll get Abilene S Style (Strat) Electric Guitar; Maple Neck, 3 Pickups, Korea, £32.48. 3 pickups = 18 polemagnets...
