Is softwood appropriate for furniture squares on a hardwood cabinet?
My suggestion is to dump the use of the 18 gauge brads. There is absolutely no reason to use anything bigger than 3/4" x 3/4" material for this type of internal joint block. Use flat head wood screws placed into pre-drilled countersunk clearance holes in these glue blocks. Then apply glue to the block faces where it meets the cabinet sides and install the screws.

It is OK to use any type of material you want for this type of glue block but I have found from practical experience that soft woods like pine are too prone to splitting when using as described above. Oak is great as it glues very well but poplar would be acceptable.