Easy atm i have a 12h pony and a 14h pony sharing a 14' x 14' box cos i have run out, they have a fence down the middle and yes it is not perfect but they are both warm and dry and happy.
Yes that's big enough for overnight or short time stabling. Horses can live in standing stalls which are smaller than this, so I'm sure you're little Sec A will be absolutely fine.
Over here there are lots of big horses who live in tiny stables, so I'm used to seeing huge horses in 8x8s and 9x9s.
As a stop gap I'd say you could use one but if you are considering having your Sec A use an 8' x 8' permanently I'd say no. A normal pony stable is 12' x 10' & that is significantly larger than the stable you mention.
My 16.3hh TB is in a 10 x 10 stable and he fits in there just fine and is incredibly cosy!! It is the only stable there is so he was just going to have to fit in there. He lies down every night to sleep and is quite happy. He also rolls but due to having big banks he doesn't get cast. I do put stable wraps on just in case he bashes his legs when he's rolling.
My stables are 9'4" x 9'4" and they have housed a BIG 16hh cob, a 16hh TB and now my 15.3 TB without any problem (able to lay down, move around) so I would say 8 x8 would be fine for a pony.