The short answer is "where are you getting your guppies from?"
If you mean store-bought guppies, then no, these would not be safe. Quite the reverse. Farmed guppies are plagued with several incurable infections including Mycobacteria and Nocardia infections that only a fool would introduce to their aquarium.
But if you are going to breed your guppies at home, from stock known to be healthy and parasite-free, then yes, once gut-loaded, such guppies could be used as food. The downside to this approach is the insane expense, and once a predatory fish becomes addicted to live fish as food, switching them to anything else can be extremely hard. There's no practical or nutritional reason to adopt this approach; your spiny eel will do at least as well on other foods, and can be thus maintained at a fraction of the cost.
If you really want to use live foods, then buy a worm farm kit. Grow your own earthworms. Earthworms are parasite-free, easy to grow in quantity on garden and kitchen scraps, and because they are gut-loaded by default, their nutritional profile is excellent.
Cheers, Neale