For shooting inside your home in an HD situation what you have is pretty close to perfect. I wouldn't put Longshot as my first choice for a light recoiling defense load, but it will do if that's all you have. I would recommend any of the Rem hulls and either the WAA12 or R12L wads with between 19 to 21 grains of Green Dot. Stacking home cast "00" by 2's (an 8 pellet load) I can get a nice even 6" to 12" pattern from 7 to 15 yards (which for HD is about perfect) using 21-22 grains of Green Dot. At those pressures it really doesn't matter what primer you use, but if you work up beyond 21 grains you'd better check your load against published data.
Now for Longshot.......that is one fast moving powder. I love Longshot for some types of loads, but it does push at both ends and makes tactical shooting more of a challenge (for me anyway). Since you didn't specify what type of buckshot you intend to use, I suggest you go
here and check out lead shot data for Longshot and Remington Premier hulls that matches the weight of your intended load.
My advice, and I am only speaking about what works best in my Mossberg 590, is not to get greedy with your lead. An 8 pellet load of "00" inside a shot cup has worked better for me than trying to cram 9 balls into the cup. If you want 9 "00" or even 9 "0" it works best to cut the petals off the wad and stack your shot in layers of three. You may need to put a hard card underneath the shot to adjust your crimp height. For these type of loads--meaning ones that don't' require a tight pattern like a hunting load does--I use cards punched out of cardboard or (my new favorite) the foam display boards like the ones kids use at school for projects. #1 Buckshot should fit in your wads in layers of three. #4 buckshot is just a matter of finding out how many pellets you can dump in the wad to get a good crimp, and then (using shot data)matching your powder charge to the weight of that amount of lead.
I shoot a bunch of low recoil HD rounds when I play "gravel pit warrior"
! I find target shot loads have enough punch to do the job, and still let you recover quickly for your next shot. Hunting loads are a whole other story........
Hope this helps.
edit: I added a chart to help you figure out the weight of your shot. Be sure to "round up" when choosing a shot load recipe.
http://data.hodgdon.com/shotshell_load.asp