viper

what elo is it appropriate to start using viper in as a secondary controller, also would u guys recommend sova and fade in low elo?

Use them if you have fun playing them

If you want to mix max it, in this current map pool I'd only consider Split and maybe Pearl. As for the rank I'd say probably ascendant to immortal if you want anywhere near the coordination to get the max potential from viper. But tbh if it's ranked you can probably get away with it on multiple maps, and as #2 said if you enjoy it you'll be fine in the long run
Sova and Fade in low elo can be very good if you use them correctly, people often don't break your util if you place is right which gives you a huge advantage

Until Ascendant agents don't really matter just play something you are comfortable with, that is self sufficient and doesn't require much coordination with your team. Viper is one of the worst for solo queue in my opinion and you can only play in high elo. Sova and fade can work.