Pretty much impossible to make sure there is a pet loop available when you make your reservation given that WDW doesn't assign sites or loops until a few days prior to arrival.
And I agree with 2Goodfycampers - if management puts me somewhere knowing that I have my dog with me (and charging me my $5 a day for his presence), I would not move unless they offered me something in return for the aggravation. If this was a big ongoing problem, they'd change their reservation system to be a lot more specific about whether or not pets are coming along and how they assign loops.
I did glean, however, that all Full hookup loops are pet loops - so I guess you could guarantee that you're in a pet loop if you book a Full Hookup site (at least at this moment before they change something around again.)