Recommendation for the best guide to hostels

Gronze.com is a good source of information

Like @JCLima, I have found Gronze to be a good resource. You will find an indication of whether a place is exclusively for pilgrims or oriented to pilgrims. Where a property does not report that, I assume that it isn't. As well, there are certain places which clearly aren't pilgrim albergues like HI and other hostels, guesthouses, B&Bs, etc. You might also find one of the Camino oriented apps useful. There are several. Of those, WisePilgrim is the one that I found easiest to use.

I have walked the Variante Espiritual, but only after I had walked the Central Route. I think it's a personal choice about what you want to see and do. Both routes have their attractions, and I wouldn't recommend one over the other.