It's good that you're looking at a little time before doing something like that. But like I said, you need time & experience in it. This kinda illustrates what I was saying.
Everything judo would offer, is found in Hung Ga. Not exactly the same, not in the same quantity, not in the same manner but it's there. All TCMA's have four foundation pieces... Shuai (Throwing), Da (Striking), Ti (Kicking), Na (Controlling). How much of each piece is kinda dependent on the methods found with in the style & it's theories. However, they're all found, all practiced, all integrated to fit the method.
hopefully his school will cover the throwing and grappling aspects of the system. I
Yes, the school does. Normally we focus on striking (It's HG after all...) but last week we did a bit of Shuai and (Chin?) Na. Higher belts sashes do it more often as far I know.
Bjj or Judo would be better options.
The lack of forms and the grappling focus won't confuse your Hun Ga training. Also the heavy emphasis on sparring within those styles will do you some good in the long run.
Yes. If I train in another MA (not yet, I want to learn at least the HG fundations at least), a grappling MA would be a good choice for the reasons you pointed. In my area I have BJJ, Judo and Shuai Jiao, an art that will fit with Hung Gar I think.