
wow, if they have sniffed each other, and walked together,
that is a very great start!

wow! that
half the battle right there!

still, some dogs feel differently in their own homes...so it's not settled completely yet.
.Might be good idea to continue to do this,
for many times, over many days,
to help the dog-aggressive dog solidify in his mind, "this here border collie is okay with me"

kinda thing. Maybe even have a yard visit play session in your yard.
.
I'd feed them in separate rooms since your dog is resource guarder for now. Has your dog-aggressive dog witnessed you fawning over the border collie yet? any reaction at all to that?
there is absolutely no sign the BC is dog-aggressive ---not at all, right? (
dog-aggression doesn't manifest til about 9 mos old, but, some ppl say they can spot it's earliest signs a month or two earlier)
No sign the dog is shy dog either, right?
cuz you don't want an insecure dog being raised side by side with a dog-aggressive dog, then you can have TWO dogs displaying dog-aggression,
and that is handful.
.
I don't know much about the toys and bones and all that stuff, short of never giving these dogs toys and bones in front of each other, but,
at some point,
that may become impractical.
but, hopefully, someone will be by to help you get off to good start with how to handle the toys and bones..and everything else.
I think you are wise to approach this efffort with all the knowledge you can gather, easier to head off/avoid an issue from ever starting, than to fix one in progress,
to get off to best possible start and all.
.
be aware,
doorways,
narrow hallways,
gates,
anything that can be perceived by the dog as a doorway,
can be a trigger for some dog-aggressive dogs.
so, hopefully, someone can guide you on exactly step by step how to bring the dog into you house that first time,
to get off to best possible start.
Like,
don't have resident dog first greet the arriving new dog IN the doorway is my point.
Maybe walk them first,
then into backyard,
let them play and hang out til tired,
and then,
hopefully,
coming in through
the doorway of his own house will not set off your dog-aggressive dog.
Doorways seem like it'd be something the dogs would sort out,
and with normal dogs, that works great. Normal dogs do sort it out. As you know, sometimes with dog-aggressive dogs, it can be whole other deal, and never does get sorted out well between the two dogs, if one is dog-aggressive...
have bucket of
water ready anyway for their first doorway thing if a
full on fight does bust out,
but yawn and stay calm
.
and who knows, it might go smooth as peaches, it sounds like these two dogs are compatible so far as you know now.
even my gangsta dog has many dogs he loves,

and can even walk through doorways or narrow hallways with his friends

.
do you have option to return the border collie if, when in the home, the toys/bones/etc becomes too much of trigger,
or
is taking that BC home a long term commitment? Can you have option to 'test drive' the two dogs together for a week or two before deciding if the new dog can fit with your resident dog? If a few months down the road, if the two dogs are not getting along, will you have option then to rehome this dog?
Are there any ADULT dogs in need of your fostering? (so you can be
sure it is not another dog-aggressive dog)
or
opposite-gender young dogs?
Even my gangsta tends to LOVE young female dogs, and once he loves them as young dogs, he loves them for life.
still, it sounds good so far

,
and this might work out like peaches and cream,
even dog-aggressive dogs have friends!! and housemates!! HOPE IT ALL WORKS OUT!
AND GOOD ON YOU FOR HELPING OUT HOMELESS DOGS!!!!!!!